The study was based on the testing of a random sample of 294 of the approximately 42,000 inmates in State prisons in 1988. The study of the sample involved a comprehensive review of inmate records, administration of the adaptive Behavior Scale and the Wechsler Adult Intelligence Scale, identification of at-risk sample inmates, and expert clinical reviews. The study found that, contrary to many estimates, a relatively small proportion of inmates are developmentally disabled (defined as persons with significant limitation in at least three of the following seven life skill areas: self-care, receptive and expressive language, learning, mobility, self-direction, independent living, and economic self-sufficiency). The battery of academic achievement tests and the Revised Beta IQ test administered to all incoming inmates is apparently unreliable in the identification of developmentally disabled inmates; however, correctional officers in the reception centers identify approximately half of the inmates with developmental disabilities for additional testing or placement in special units. The vast majority of the inmates with developmental disabilities are "mainstreamed" into the general population of State prisons. Approximately 10 percent of the inmates with developmental disabilities are housed in two special units reserved for inmates who are determined, based on their disabilities and behaviors, to be at high risk of harm if placed in the general population. Recommendations for the correctional management of developmentally disabled inmates are offered. 15 figures, 41-item bibliography, and appended study instruments