Describes changes brought about by the Sentencing Reform Act of 1984 in sentences imposed and time served in prison by cohorts of offenders entering and exiting Federal prison between 1986 and 1997. During this period, average prison sentences increased from 39 months to 54 months. The proportion of the sentence to be served by offenders entering Federal prison increased from 58% during 1986 to about 87% during 1997. Overall, time to be served increased from 21 months, on average, for those entering Federal prison during 1986 to 47 months for those entering during 1997. Time to be served for those entering prison increased from 23 to 75 months for weapons offenders, from 30 to 66 months for drug offenders, and from 74 to 83 months for bank robbery offenders. For those released from Federal prison, time served increased from 15 months, on average, during 1986 to 29 months during 1997.