To determine whether high plasma cortisol concentrations are a distinctive feature of depression or whether plasma cortisol is also elevated in other forms of psychosis, cortisol concentrations were measured in 59 patients with acute functional psychoses, six non-psychotic depressed patients and 37 control subjects, all free of antidepressant and neuroleptic drugs for at least three months. Patients with schizoaffective disorder, manic type, had the highest concentrations throughout the day and those with major depressive disorder, psychotic sub-type had higher concentrations than controls in the afternoon and evening. Manic and schizophrenic patients had cortisol concentrations above controls in the afternoon only. Elevated concentrations were not related to the presence of depressed mood or to duration of stay in hospital, and a return to normal occurred irrespective of the type of treatment used. Thus raised plasma cortisol concentrations are a feature of psychotic illness, but in drug-free patients are not specific for severe depression.