A theory of religious conversion, social attachment, and social learning guides this study of prison religion and its influence on the rehabilitation of adult male offenders. The study found the religious involvement of inmates in a large medium/maximum security prison was extremely varied and extensive. During a 1-yr period 49% of the inmates attended at least 1 religious service or program. Over 800 religious services or meetings, across many different denominations and religious groups, were held during the year. Two prison chaplains, 4 inmate religious clerks, and 232 volunteers who donated about 21,316 hours of work to the prison made this high level of programming possible. The estimated yearly cost of these religious services was inexpensive at between $150 to $250 per inmate served; in contrast, other effective correctional programs cost around $14,000 per person. Logistic regression found an inverse relationship between intensity of religious involvement and the presence or absence of in-prison infractions. As religious involvement increased the number of inmates with infractions decreased. Findings provide greater insight into the nature of religion in prison setting and support the view that religion can be an important factor in the process of offender rehabilitation. (PsycINFO Database Record (c) 2019 APA, all rights reserved)