HMP Grendon, located near Aylesbury, England, is a specialist prison operated along the lines of a therapeutic community. Prisoners selected for Grendon between 1984 and 1989 tended to be high-risk offenders, relative to other prisoners of comparable age, and were serving similar sentence lengths for similar offenses. Lower reconviction rates were found for prisoners who went to Grendon than for prisoners selected for Grendon but who did not go. Time spent at Grendon was strongly related to reconviction; reconviction rates were lower for prisoners who stayed for longer periods. Prisoners who stayed 18 months at Grendon exhibited reductions in reconviction rates of about 20 to 25 percent. Both mode of release from Grendon, i.e., transfer back to the prison system or release into the community, and length of stay at Grendon had an impact on reconviction rates. Of the two, however, length of stay seemed considerably more important. Treatment effects for sexual and violent offenders were less clear but, for those who stayed at Grendon for longer periods, there was some reduction in reconviction rates for sexual and violent offenders. 5 references, 2 tables, and 3 figures