Analyses of post-hearing questionnaires and case files from 1,035 actual parole decisions revealed that the Parole Board considers institutional behavior and predictions of future risk and rehabilitation in the decision to release on parole. Predictions seem also to be based on diagnostic judgments identifying causes of crime such as personal dispositions, drugs, alcohol, money, and environment. A one-year follow-up of 838 released parolees showed that predictions were virtually unrelated to known post-release outcomes. An actuarial prediction device was developed that is more predictive than subjective judgments. The use of decision guidelines to structure discretion is discussed, as well as the utilization of our research in guideline development by Pennsylvania. (Publisher abstract)