In a prospective double-blind trial we examined the affective morbidity and side-effects of 72 patients who were randomly allocated either to continue with their usual dose of lithium or to receive either a 25% or 50% reduction in lithium dosage. Patients who underwent a dosage reduction with consequently lower plasma lithium levels (0.45-0.79 mmol/l) had significantly decreased affective morbidity. Thyroid stimulating hormone levels were also significantly decreased in this group. Total subjective side-effects score and tremor were also reduced. No change in affective morbidity was observed during the trial in patients whose dosage was not altered. These changes were observed in both unipolar and bipolar patients. It was concluded that a once a day dosage with a sustained release lithium preparation that maintained a 12-h plasma level of about 0.6 mmol/l is both more effective and produces less side effects than does conventional dosages.