SummaryThe method of backcalculation was used to estimate the cumulative number of human immunodeficiency virus type 1 (HIV-1)-infected adults in Germany from reporting delay-corrected surveillance data on acquired immunodeficiency syndrome (AIDS) in the pretreatment era. Using different back-calculation approaches with various incubation period distributions, a plausible range of 13,100 to 23,900 HIV-1-infected adults as of December 31, 1984, was calculated. Estimates of the number infected at more recent times were subject to much greater uncertainty. On average, the cumulative incidences calculated by the nonparametric backprojection method are about 15% lower than the results from the step function model. Nonparametric backprojection with the Hessol incubation distribution suggests a declining HIV infection rate after 1985, as might be expected from German health policies. This distribution is derived from cohorts of homosexual men, the main fraction of German AIDS cases.