A group of patients with major depressive disorder, with and without comorbid obsessive-compulsive disorder, completed the Tridimensional Personality Questionnaire (TPQ). Harm Avoidance scores were found to be high compared to published age-matched norms and to display a significant positive correlation with Hamilton Depression Rating Scale scores. Platelet 125I-lysergic acid diethylamide (125I-LSD) and 3H-paroxetine binding Bmax values were measured to test Cloninger's hypothesis that Harm Avoidance scores would correlate significantly with measures of serotonergic function. A significant inverse correlation was found between Harm Avoidance scores and 125I-LSD Bmax values. Correlations between 3H-paroxetine Bmax values and TPQ scale scores were not significant. These results suggest an alternative view of the literature relating platelet 5-hydroxytryptamine-2a receptors and mood disorders in that the temperament dimension, Harm Avoidance, may explain prior inconsistencies involving links with depression and suicidality.