Recent high-profile terrorist attacks have led to attempts by social scientists to investigate the processes behind radicalization. Prisons have been identified as possible breeding grounds for radical extremism. However, the evidence so far is based almost solely on case studies. The research provides one of the first quantitative assessments of prison radicalization with directly measured extremist attitudes among detained terrorism suspects. The findings suggest that the prisoners indeed radicalized over time. This trend was predicted partially by demographic variables such as marital status, and psychological factors such as the need for cognitive closure (NFCC) and social dominance orientation (SDO).