Institutional intelligence test records were examined in identifying 436 prisoners with IQ's below 86 among 3,938 adult male offenders incarcerated in Maine and Massachusetts penal institutions. Mentally retarded and borderline mentally retarded inmates were included in the study group. Legal characteristics investigated included severity of offense, severity of first offense, sentence length, recidivism, juvenile history, probation history, and participation in rehabilitation programs. Social variables included race, age, disabilities, educational status, marital status, employment history, reported character disorders, and substance abuse history. Results showed that mentally retarded offenders tended to be single white males who had been employed in low-skill jobs. Most had been involved in special education programs. Over half of the participants had alcohol abuse problems. First offenses were moderately severe, and tended to be less severe than current offenses. Average sentence was about 18 years; when life sentenced offenders were excluded, the average sentence length was 8.9 years. Retarded offenders were more likely to differ from borderline offenders in terms of social characteristics than they were in terms of legal characteristics. Results indicated that the incidence of mentally retarded offenders was only slightly higher than the rate of mental retardation in the general population. Results also suggested that the social and legal profile of retarded offenders is similar to offenders of borderline intelligence. Although social and legal variables were better predictors of offense severity, sentence length, and past recidivism than was intelligence, none of the predictors was particularly strong. Tables, reference notes, and 15 references are provided.