The LDS address unknown file or "lost address file" consists of church members who cannot be located. At times, previously baptized members move and cannot be located by their local congregation. In these cases, home teachers or local leaders seek to inquire regarding the new location so that membership records may be forwarded to the appropriate church unit. If the member cannot be located, his records are sent to Salt Lake City and become part of the "lost address" or "address unknown file." Church clerks make efforts to locate the individuals by attempting to reach acquaintances or known contacts. Many times, these efforts are successful, in which case the records are transferred to the new congregation. If efforts to locate the individual are unsuccessful, the individual remains on the lost address file until the age of one hundred and ten years or until proof of death can be located. The average US life expectancy is approximately 78 years, and that in Latin American nations is significantly shorter, and so it is possible that some individuals may continue to be counted on membership rolls for decades after their death. The intent of this policy is to make sure that living members are not mistakenly removed from membership rolls and that ongoing efforts are made to reach them, even if this may result in some inflation of membership numbers by the inclusion of some lost members who have deceased.
According to Elder Merrill Bateman, the "address unknown file" for Utah consists of 180,000 names, or 10 percent of LDS membership in the state. He reported that approximately 50,000 individuals in Utah are added to the lost address file each year. Ninety percent of those are found within the next year, while those on the list longer than one year (and located less frequently) constitute over 70 percent of lost address file members. Elder Bateman acknowledged that many of the members on the "lost address file" list are "less-active," especially those on the list for more than twelve months.
Former Chilean mission and MTC president Ted Lyon reported that 200,000 of the 535,000 nominal members in Chile -- over 37 percent -- are in the "lost address" file. Data on unknown address file membership in other areas has not been released. In view of broadly similar data on LDS membership self-identification from national censuses and other sociological indicators in Latin American nations, it seems reasonable to hypothesize that rate of "lost address" members in these nations is similar to Chile. Mexico and Brazil fared similar to Chile on the national census when age-based criteria are included, as I will demonstrate later. Reducing this estimate to 30% for Latin America as a whole would appear to be a fairly safe and conservative figure. It also seems likely that the 10% of LDS members on the "address unknown" file in Utah, where most of the population is LDS and many lost members can be located through family contacts, represents a "best possible case," and that the percentage of lost members in other states and Europe is likely to be at least this high or somewhat higher. As approximately half of LDS members live in developed nations and half in developing nations, application of the very conservative "address unknown file" estimate of 10% for developed nations and 30% for the developing world would lead us to conclude that no fewer than 20% of LDS members worldwide are on the "lost address file," or 2.6 million of the 13 million members. The true number may be as high as 25-28%.
The "address unknown file" poses special difficulties to researchers in the calculation of LDS member participation rates. "Address unknown" members are included in country and overall LDS membership statistics, but are not included in congregational membership rolls. However, weekly church attendance rates are calculated based on number of members on congregational membership rolls, and do not consider "lost address file" members. As a result, reported LDS statistics for average weekly sacrament meeting attendance substantially overestimate actual member activity when lost members are added back into the denominator.
For instance, let us suppose that two hundred thousand members were on the "lost address file" in a nation with one million church members. If two hundred thousand of the eight hundred thousand members on congregational rolls attended church weekly, the weekly church attendance rate would be reported at 25%. Yet when the two hundred thousand lost members are added back into the denominator, the actual weekly church attendance rate is two hundred thousand out of a million members, or 20%. In areas where few members are on the "address unknown" file, the impact is only modest, whereas in areas with many lost members, the discrepancy between congregational attendance rates and overall member attendance rates can be dramatic.
 Canham, Matt, "Church Won't Give Up on 'Lost Members,'" Salt Lake Tribune, October 17, 2005.
 Stack, Peggy Fletcher, "Building Faith. A Special Report: The LDS Church in Chile," Salt Lake Tribune, March 31, 2006.