A cross-sectional blind study was conducted in the spring of 1988 to estimate the extent of human immunodeficiency virus (HIV) infection among inmates entering the California correctional system. Of the 6,834 inmates receiving entrance physical examinations during the study period, 6,179 (90.4%) had serum tested for the presence of HIV antibodies after routine blood work was completed and personal identifiers were removed. Seroprevalence was 2.5% (95% confidence interval, 2.1% to 3.0%) among the 5,372 men tested and 3.1% (95% confidence interval, 2.1% to 4.5%) among the 807 women tested. Seroprevalence was more than twice as high among men arrested in the San Francisco Bay Area as in those arrested elsewhere in the state. The regional differences in HIV seroprevalence observed among entering inmates mirror infection rates reported among intravenous drug users from the same regions.