OBJECTIVE: The purpose of this study was to compare the pattern of affective psychopathology in families ascertained for genetic linkage studies through bipolar I probands to that in families ascertained through bipolar II probands. METHOD: All available first-degree relatives (N = 266) of 48 bipolar I and eight bipolar II probands were interviewed with the Schedule for Affective Disorders and Schizophrenia- -Lifetime Version by one of two psychiatrists who had attained high interrater reliability for bipolar II disorder and other diagnoses. RESULTS: Bipolar II disorder was the most common affective disorder in both family sets. Forty percent of the 47 first-degree relatives of the bipolar II probands and 22% of the 219 first-degree relatives of the bipolar I probands were diagnosed with bipolar II disorder. On the other hand, only one bipolar I relative was found in the bipolar II families. CONCLUSIONS: Bipolar II disorder was the most prevalent affected phenotype in both bipolar I and bipolar II families and was the only expressed phenotype in half of the bipolar II families. This suggests that bipolar II disorder is genetically related to but less complex than bipolar I disorder. Accurate diagnosis of bipolar II disorder may be crucial in finding the genetic loci underlying bipolar disorders generally.