Data were collected by the Federal Bureau of Prisons during three, 1-day counts in September 1986, September 1987, and September 1988. Data indicate that the rapid increase in Federal prison populations is largely driven by the increase in the number of drug law violators being incarcerated. By September 1988, approximately 44 percent of all Federal prisoners had been incarcerated for drug law violations. Of those sentenced to Federal institutions, 44 percent had never been committed to prison previously. Drug offenders were most likely and violent offenders least likely to have no prior commitments. Data on offenders' histories show that a record of violence is rare among offenders incarcerated for the first time. This is true for all categories of offenders, including drug law violators, among whom more than 90 percent of "first timers" have no history of violence. Even among drug law violators with prior commitments, the majority (60 percent) also show no history of violence. 6 figures.