A patient with bipolar disorder and previously undiagnosed Klinefelter's syndrome presented with acute mania refractory to pharmacotherapy and was successfully treated with electroconvulsive therapy. This case constitutes the 31st case report documenting the coexistence of bipolar disorder with chromosomal anomalies and the 14th such patient with Klinefelter's syndrome. The relevance of this case to understanding the genetics of bipolar disorder is explored through a review of previously published reports of bipolar patients with chromosomal abnormalities. The relatively high incidence of Klinefelter's syndrome among this group of patients is discussed in the context of evidence for linkage of bipolar illness to the X chromosome.