The purpose of this study was to determine the co-occurrence of DSM- III-R personality disorders (PDs) with mood, anxiety, and substance use disorders in a young adult population. The members of the Northern Finland 1966 Birth Cohort Project, living in the city of Oulu with an age of 31 years (N = 1,609) were invited to participate in a two-phase field study. The SCID I and II were used as diagnostic instruments. One hundred and seventy-seven out of 321 interviewed subjects met the criteria for mood, anxiety, or substance use disorders. Altogether 72 (41%) of the subjects with an Axis I disorder met the criteria for at least one PD. The weighted co-occurrence rate of any PD varied from 28% for mood disorders to 47% for anxiety disorders. PDs, especially those in Cluster C, are highly associated with Axis I psychiatric disorders in population.