This study examined the prevalence and predictors of male inmates’ mental health problems. Data were used from the Prison Project, a longitudinal study in which 824 prisoners were surveyed twice (three weeks and three months after arrival in prison). Compared with the general population, inmates reported significantly more psychological problems. Except for depressive symptoms, inmates’ mental health problems decreased over time. After controlling for prior mental health problems and a number of import factors, we found that those who shared a cell in the first weeks of their imprisonment and held more positive judgments regarding daytime activities and the relationships with staff, reported fewer psychological problems after three months. Those who were verbally abused by prison staff during their first weeks in prison reported more psychological problems after three months.