The Nation's prison population grew by almost 40,000 during the year, the largest increase in a single year since data became available in 1925. This huge 12.1-percent influx pushed the number of Federal and State prisoners up to 369,009 and surpasses the record 10.5-percent annual growth rate set in 1975. Between 1980 and 1981, the incarceration rate of sentenced prisoners rose from 139 to 154 per 100,000 U.S. resident population. During 1981, the number of States under court orders to reduce overcrowding rose from 29 to 31, while the number involved in litigation about overall prison conditions increased from 32 to 37. The 1981 prison population increase affected both Federal and State correctional systems, as well as both sentenced and unsentenced prison inmates. The number of Federal prisoners grew by nearly 3,800, or 16 percent, reversing a 3-year decline begun in 1978 that had reduced the Federal prison population by one-fourth. Most increases occurred in State institutions, which held an additional 36,000 prisoners at yearend. Behind this growth were measures reflecting a sterner public attitude toward crime and criminals. During the past 5 years, 37 States have passed mandatory sentencing laws, and 11 States have passed determinate sentencing laws, both of which frequently result in a longer average time served than do indeterminate sentences. During 1981 prison populations increased in 49 States and the District of Columbia. The number of female inmates rose by 16.1 percent in 1981, more than the increase for males and much more than the increase for women a year earlier. Four tables, two figures, and notes are provided.