Abstract The Guilty But Mentally Ill (GBMI) statute was enacted in 1975 largely in response to public calls for a decrease in insanity acquittals and for ensuring the treatment of mentally disordered offenders. The statute has been the subject of much debate since its inception, but very little empirical research has been done regarding the clinical basis of the underlying construct. The present study examined psychological characteristics of GBMI offenders and contrasted these and discharge diagnoses across the GBMI group, a psychiatric inpatient group, and a group of convicted felons not referred for inpatient treatment during the four years of the study. Results supported the hypothesis that GBMI offenders were measurably mentally disordered. However, GBMI offenders were not found to be significantly different from the general convicted population or from a prison inpatient group except on one MMPI scale. Implications for treatment and further research are presented.