The incubation period of AIDS is a key characteristic in understanding the HIV and AIDS epidemic, both clinically and epidemiologically. The incubation period distribution (IPD) provides information about the probability of progression to AIDS as it changes with time since infection with HIV. The IPD also provides the link between the HIV infection rate and the occurrence of AIDS cases over time, and is an essential feature in back calculation procedures (Brookmeyer and Gail 1988). Knowledge of the IPD creates the opportunity to make more reliable projections (Hendriks et al. 1992), which are necessary for health-care planning.

Cohort study data relating to development of AIDS are inevitably incomplete in dates of seroconversion (infection) or development of AIDS, or both. This incompleteness has inspired a variety of approaches. We used a multiple imputation procedure, with four related models, each covering different assumptions, to investigate the sensitivity of the estimated IPD regarding the imputation method. The imputation procedure was used to provide the unobserved interval between seroconversion and enrolment for those individuals who were already HIV infected at enrolment. We can exclude observations relating to individuals who received antiviral and or prophylactic treatment designed to delay the onset of AIDS. The results obtained from data such as these will be valuable in future to aid the understanding of the effects of new therapies on the evolution of the AIDS epidemic.

The IPD was estimated using data available at February 1990 from all homosexual and bisexual men with HIV seropositive blood samples (n = 348; aged 25–45 years), who were part of a larger cohort study in Amsterdam.