Prison populations are apparently near an equilibrium point from a cost-benefit perspective. The most readily measured benefit, reduced monetary loss to victims, is some $19,000 per additional inmate per year. This is substantially less than the most readily measured cost, $29,000 for prison operation and construction, less probation supervision costs. Reduction in psychological costs to victims, estimated to be worth $18,000 per inmate, raises the benefits to $37,000. For all practical purposes, given the uncertainties involved, especially for psychological costs, there is no indication that the direct calculable cost ($29,000) and benefits ($37,000) of imprisonment differ appreciably. Additional costs and benefits that are not quantified, such as suffering by victims' and inmates' relatives, also appear to be roughly balanced. Reducing crime by expanding prisons is unlikely to be cost-effective unless accompanied by greater efforts to imprison the most active and dangerous criminals. Legislators, therefore, should aim to improve police effectiveness as a way to make better use of prisons. 7 notes, 1 table, and 21 references