We evaluated the dexamethasone suppression test (DST) as a predictor of response to drugs and placebo in 105 patients, in a large double-blind placebo-controlled out-patient trial to determine the efficacy of paroxetine HCl, a selective serotonin reuptake inhibitor, compared with that of imipramine HCl and placebo. The presence of a positive or negative DST did not predict response to either paroxetine or imipramine. However, a positive DST predicted a poorer response to placebo: only 3 out of 18 patients who showed DST non-suppression responded to placebo, as opposed to 11 out of 21 who exhibited DST suppression (P less than 0.05). A positive DST was associated with a 61% response to drugs and a 16% response to placebo. This finding suggests that the presence of a positive DST implies the need for active somatic treatment.