Examined the hypothesis that imprisonment leads to cognitive impairment. The author presents a review of empirical studies and data of a research project carried out in Belgian prisons. In this latter project the Progressive Matrices were administered individually to 90 inmates in three Belgian prisons. All Ss were male, but differed in age, type of crime, and length of incarceration. Regression analysis was carried out to determine the effect of length of imprisonment on PM IQ-scores. Regression analysis was repeated for subgroups of offenders (against persons and against property). R2 and beta values never reached levels of statistical significance, which indicates that intellectual functioning was not influenced by length of imprisonment.