Of 50 women charged with crimes of violence, 44% committed their offence during the paramenstruum (P less than 0.02) and there was a significant lack of offences during the ovulatory and post-ovulatory phases of the menstrual cycle (P less than 0.01). This association could not be accounted for by psychosocial factors. Offences were unrelated to symptoms of premenstrual tension. When considering treatment, recurrent behavioural changes rather than subjective symptoms should be looked for.