Patients with chronic pain syndromes are commonly depressed. Chronic pain populations also contain distinct subgroups of personality profiles as defined by the MMPI. To assess the relevance of personality subtype to affective disorder we determined the relationship of psychiatric diagnoses defined by Research Diagnostic Criteria (RDC) to MMPI subgroups in a sample of hospitalized patients with predominantly chronic low back pain. RDC psychiatric diagnoses for the sample were major depression (44.2%), minor depression (19.2%), other psychiatric disorder (13.5%), and no mental disorder (21.6%). Patients satisfying RDC criteria for major depression were significantly associated with discrete MMPI personality subtypes. No other psychiatric diagnoses were significantly associated with distinct personality subgroups. No relationship was observed between personality profile and presence of demonstrable organic etiology for pain. These findings indicate that behavioral and pharmacological interventions directed at depression as well as pain are important in the treatment of chronic pain populations, especially in selected subgroups.