The results did not support the hypotheses of the study, which expected that there would be a worsening over time in reported behavior/sensations and that this change would be worse for inmates with mental illness in administrative segregation (AS). However, significant changes were found to occur over time and they tended to be in the direction of improvement and the improvement tended to occur more frequently for inmates with mental illness. When comparisons of the AS groups were made to the relevant comparison groups, there was no indication that the segregation groups behavior and attitudes declined over time in comparison to the non-segregated groups. One of the most widely debated topics in the field of corrections, the use of long-term AS, has suffered from a lack of empirical research, with critics arguing that the conditions of AS confinement exacerbate symptoms of mental illness and create mental illness where none previously existed. This study supported by the U.S. Department of Justice, National Institute of Justice tested three hypotheses: (1) offenders in AS would develop an array of psychological symptoms consistent with the SHU syndrome, (2) offenders with and without mental illness would deteriorate over time in AS, but at a rate more rapid and extreme for the mentally ill, and (3) inmates in AS would experience greater psychological deterioration over time than comparison groups. Study participants included only male inmates at the Colorado State Penitentiary who were placed in AS and comparison inmates in the general population. Tables, figures, references, and appendixes A-C