To measure the impact of the 1987 expansion of the definition of acquired immune deficiency syndrome on the number and characteristics of cases in the United States, we reviewed the 28,920 cases diagnosed since the revision and reported through 1988. The proportion meeting only new criteria was 28% overall, with a range of 0 to 82% in different states and territories. This proportion was 26% in the last quarter of 1987 and increased to 31% in the last quarter of 1988. It was higher in heterosexual intravenous drug abusers (IVDAs) (43%) and lower in homosexual male non-IVDAs (21%) than in other groups. The new criteria, but not earlier (pre-1985) criteria, generally require a positive test for human immunodeficiency virus (HIV) infection; however, use of HIV testing varies among states, as demonstrated by differences in the percentage of pre-1985-criteria cases with a reported HIV test (39 to greater than 95%). The revision has changed the distribution of characteristics of cases (e.g., heterosexual IVDAs composed 18% of cases meeting old criteria, 35% of cases meeting only new criteria, and 23% of all cases). Interpretation of trends in both the number and characteristics of cases should take into account the variable impact of the revision on reporting.