To our knowledge, this is the first study to examine the systematic comorbidity of DSM-III-R personality disorders (PDs) in a sample of alcoholic outpatients. The extent and direction of overlap can provide a measure of heterogeneity and descriptive validity. Fifty sober alcoholic outpatients enrolled in a treatment program were assessed by Structured Clinical Interview for DSM-III (SCID) instruments for the presence of axis I and axis II disorders; 80% had either a coexistent axis I or II disorder, with 66% having an axis I disorder, 64% an axis II disorder, and 50% both axis I and II disorders. There were 84 PD diagnoses among the 32 PD patients (2.6/patient), with multiple diagnoses in 20 (62%). The most prevalent PD diagnoses were paranoid (44%), antisocial (20%), avoidant (20%), passive-aggressive (18%), and borderline (16%). Overlap was extensive and not confined to any one of the three designated axis II clusters. Poorer outcome was associated with the presence of PD. Personality variables may offer a means of further characterizing the heterogeneity observed in axis I disorders. Further refinement of the current system of PD classification and investigation into alternate models is needed.