We found a 38% lower maximal prolactin response to an oral challenge dose of 60 mg of dl-fenfluramine relative to placebo in younger (< 30 years) depressed inpatients compared with the response in age-matched healthy controls (p < .03). Severity of depression did not correlate with prolactin response. Prolactin responses in older depressed patients (> or = 30 years) did not differ from older controls. Younger depressed patients differed from older depressed patients in terms of earlier age of onset of first lifetime episode of major depression, greater degree of suicidal intent during a recent suicide attempt, double the level of hopelessness on admission to hospital, and a higher rate of comorbid borderline personality disorder. A blunted prolactin response to fenfluramine may be interpreted as evidence for reduced serotonergic function in younger depressed patients and may underlie their observed greater suicidality and hopelessness.