Cross-sectional analysis of 441 individuals with bipolar disorder treated at a US health maintenance organisation investigated the distribution of manic and depressive symptomsin that illness. Clinically significant depressive symptoms occurred in 94.1% of those with (hypo)mania, while 70.1% in a depressive episode had clinically significant manic symptoms. DSM-unrecognised depression-plus-hypomania was over twice as prevalent as DSM-recognised mixed episodes. Depressive symptoms were unimodally distributed in (hypo)mania. Depressive and manic symptoms were positively, not inversely, correlated, and their co-occurrence was associated with worse quality of life. Implications for the DSM and ICD nosological systems are discussed.