In Portugal, offenders found not guilty by reason of insanity (NGRI), may be given a restricted order to a special hospital as an alternative to prison. In European countries there is a recognized need for data concerning this special population. The aim of the present study was to examine the characteristics of all the NGRI subjects (n = 274) detained in the country in a descriptive and retrospective survey conducted in January 2009. Offence committed, demographic factors, diagnosis at admission, background of substance abuse and diagnostic stability were recorded. Schizophrenia was the commonest diagnosis (51.5%). Mean population age was 42.6 years, with only 6.2% women. Homicide was the most common offense (41.2%). A background of substance abuse was found in 42.3% of subjects. There were significant differences in the schizophrenia and mental retardation patient groups when compared individually with the other diagnoses concerning homicide and arson as the offence. Mean duration of inpatient stay did not differ significantly between diagnoses. The findings also point to poor follow-up of the NGRI patients after admission.