Psychosis is reported to show a later age of onset in women than in men and its nature and course in women may also differ. The purpose of this study was to determine if levels of four steroid hormones at the start of early onset psychosis differ from the levels of other groups of young people and if predicted low levels of estrogen (E2) are a feature of female psychosis. Two blood samples from 22 young psychotic patients were analysed by radioimmunoassay for E2, progesterone (PROG), testosterone (TE), and dehydroepiandrosterone sulphate (DHEAS). Female psychotic patients showed E2 levels lower than matched healthy cycling controls but higher than those on a contraceptive pill; they also showed higher TE levels than controls. Male psychotic patients had higher DHEAS levels than healthy or obsessive compulsive disorder (OCD) subjects. We suggest that illness-related changes of steroids can be measured superimposed on medication-induced changes and that lower E2 levels in psychotic women may increase their vulnerability to psychosis. Changes of TE in female and DHEAS in male psychotics may be more a consequence of the illness.