In a sample of over 2,500 medical charts from inmates arrested in 1986 in New York City, the rate of intravenous drug use peaked at 43% in male inmates aged 36 to 40. Younger inmates had progressively lower rates of intravenous drug use. Inmates detained for longer time periods and female inmates had particularly high rates of intravenous drug use. Rates of intravenous drug use were 30% among Hispanic, 26% among White, and 16% among Black inmates. Although overall rates of drug use among all male inmates in 1986 have remained unchanged since 1975, drug use was most prevalent in inmates aged 31 to 40 in 1986, the same criminal “cohort” which in 1975 had the highest rate of drug use. Implications for targeting of AIDS and drug-related counseling among inmates are discussed.