Objective: Although previous studies have revealed an association between androgens and aggression, there is a lack of knowledge of this issue in different mental disorders.

Method: The associations between serum testosterone levels and criminal behaviour in different mental disorders were examined using data collected from forensic psychiatric male patiens (20 schizophrenics and 42 subjects with personality disorders).

Results: Male criminals with personality disorders had significantly higher serum levels of total and free testosterone than criminal schizophrenics. Among schizophrenics, total (P= 0.01) and free testosterone (P = 0.01) declined significantly more with age compared to healthy controls and patients with personality disorders, and also correlated with duration of neuroleptic drug use (r=‐0.60, P= 0.000 for total and r=‐0.46, P = 0.0001 for free testosterone). The recidivists with personality disorder had higher total (P=0.04) and free testosterone (P=0.05) levels than non‐recidivists with personality disorder.

Conclusion: Personality disordered criminals with multiple offences had high serum testosterone levels. The low testosterone concentrations among schizophrenics may be due either to long‐term use of neuroleptic agents or to the mental illness itself and its possible inhibition of the pituitary‐gonadal axis.