Presented in part at the Third International Conference on Bipolar Disorder, Pittsburgh, June 17–19, 1999. Received Jan. 3, 2000; revision received May 22, 2000; accepted July 5, 2000. From the Department of Psychiatry and Behavioral Sciences, Johns Hopkins School of Medicine, Baltimore; the Department of Biostatistics, Johns Hopkins School of Hygiene and Public Health, Baltimore; the Department of Psychiatry, University of Chicago, Chicago; and the National Naval Medical Center, Bethesda, Md. Address reprint requests to Dr. Potash, Meyer 3-181, Johns Hopkins Hospital, 600 N. Wolfe St., Baltimore, MD 21287-7381; firstname.lastname@example.org (e-mail). Supported by grants from NIH, the National Association for Research on Schizophrenia and Depression, the Alexander Wilson Schweizer Fund, the Charles A. Dana Foundation Consortium on the Genetic Basis of Manic-Depressive Illness, the Theodore and Vada Stanley Foundation, the American Foundation for Suicide Prevention, and Mrs. Helen Chaney. The authors thank Paul McHugh and Kay Jamison for their help, Jo Thomas for data management, Barbara Schweizer for family recruitment, and the study families for their time and energy.