The study defined "sexual reconviction" as "conviction for another sexual offense during a specified follow-up period." The definition of "sexual offending" excluded any sexual offenses considered to be consensual or victimless. The study identified two cohorts of sex offenders discharged from prisons in England and Wales in 1992 and 1993, using the Prison Service database. Offenders who had participated in the Prison Service sexual offender treatment program were excluded from the samples. The final samples consisted of 555 offenders in the 1992 cohort and 535 offenders in the 1993 cohort. It was assumed that untraced offenders would have the same risk of reoffending as traced offenders. The results were compared with sexual reconviction rates for discharged sex offenders from previous studies in England and Wales. The study found that the 2-year rate of sexual reconviction had decreased to produce a low base rate of sexual recidivism. The 4-year rate of sexual reconviction declined substantially from 12 percent in 1980 to 5 percent in 1992 and 1993. The low base rate of sexual reconviction for discharged sexual offenders who had not received treatment presented a serious methodological problem in evaluating prison-based treatment programs for sex offenders. A "successful" treatment program would reduce the rate of reconviction by some statistically significant degree; however, if the rate of reconviction is already low, a reduction would be so small as to make it difficult statistically to show that this difference was due to the treatment effect rather than to chance factors. The authors propose new outcome measures, such as police cautions and arrests, parole performance, data from social service files on contacts with children and reports of child abuse, and police intelligence information on registered sex offenders. A pilot research study that uses various outcome measures is recommended. 1 figure, 1 table, and 24 references