Data from bipolar I old-order Amish families suggest that the morbid risk of illness is not significantly different in this population when compared with estimates of risk from previous studies. The age-corrected rates of bipolar I, bipolar II, and major depressive disorder among first-degree relatives are 8.7, 3.7, and 11.6, respectively. Risk of illness is not significantly different among male and female relatives and among relatives of male and female probands. Consistent with other reports, the rate of illness is higher among relatives of probands with early-onset disease. In contrast with previous reports, there does not appear to be a cohort effect in this population.