Previous theoretical developments in the field of prison culture have produced the concept of prisonization, which means the identification with the folkways, mores, customs, and general culture of the penitentiary. The prisonization hypothesis holds that minimal levels of prisonization are felt during the first and last 6 months of incarceration, with the maximum levels of prisonization felt by inmates during the middle portion of their incarceration. The author asserts that the prisonization hypothesis is in need of revision, and as such, this study assessed criminal thinking and identity in 148 Federal prison inmates housed in a medium-security institution. Fifty-five of the inmates had no prior prison experience and no more than 6 months in prison total (novice), while the remaining 93 inmates had at least one prior prison experience and a total of at least 5 years in prison (experienced). Participants completed Self-Assertion/Deception scale of the Psychological Inventory of Criminal Thinking Styles (PICTS) and the Centrality subscale of the Social Identity as a Criminal (SIC) questionnaire. Six months after the initial assessment, participants were re-evaluated using the same measures. It was hypothesized that novice inmates would display an increase in criminal thinking and identity when re-evaluated at the 6-month follow-up. Results indicated support for the hypothesis that aspects of criminal thinking and identity increase in novice inmates imprisoned in a medium-security relative to experienced inmates in the same environment. The author posits that prisonization may be an adaptive response to an unfamiliar environment that becomes maladaptive upon release. One limitation of the current study is its questionable generalizability and clinical utility. Appendix, notes, references