Of 26 patients with active Cushing's syndrome assessed before and at three and 12 months after treatment, 21 had pituitary-dependent disease. Median urinary free cortisol values (per 24 hours) were 680, 180 and 200 nmol at zero, three and 12 months (normal less than 270 nmol), with significant improvement (P less than 0.001) at three and 12 months. Depression on the Hamilton rating scale was significantly less at three months (P less than 0.01) and at 12 months (P less than 0.001). We have already demonstrated that some patients with Cushing's syndrome have PSE diagnoses of depression and are more depressed than patients with other pituitary tumours. This is the firmest evidence to date that when Cushing's syndrome occurs it commonly causes depressive illness.