To assess the immediate impact of the proposed CD4-based expansion of the AIDS case definition, we determined two key proportions from a subsample of men from the San Francisco City Clinic Cohort (SFCCC). We then used Bayes theorem to project the number of persons fitting the proposed definition in the entire SFCCC and in the city of San Francisco. Among those men meeting the 1987 AIDS case definition, the proportion with a CD4 cell count < 200 cells (within 6 months of their AIDS diagnosis) is 0.70 (16 of 23). Among those with a CD4 count ever < 200 cells, the proportion with AIDS is 0.40 (29 of 73). Our estimates show that 446 persons in the SFCCC and 3,603 persons in San Francisco would fit only the expanded definition. Thus, the proposed definition would likely more than double the number of persons who could be diagnosed with AIDS. Bayes theorem offers a simple method for estimating the immediate impact of the proposed CD4-based expansion of the AIDS case definition.