The exact prevalence of bipolar disorder in late life is uncertain. Based on four large-scale studies that used very different sampling methods, the prevalence of bipolar disorder in the community has been generally reported to range from 0.08% to 0.5%. The Epidemiologic Catchment Area (ECA) study conducted in the early 1990s sampled 18,263 community-dwelling Americans at five sites to determine the prevalence of mental illnesses for those ages 15 years and older (Weissman et al. 1988). The ECA study found that bipolar disorder for adults ages 65 years and older had a 1-year prevalence range of 0.0%–0.5% with a cross-site mean of 0.1%. This was markedly lower than the prevalence among young (ages 18–44 years; 1.4%) and middle-age (ages 45–64 years; 0.4%) adults. Similarly, a large health maintenance organization (HMO) administrative database review containing almost 300,000 unique individuals found a prevalence of 0.25% for bipolar disorder in persons ages 65 years and older and a prevalence rate of 0.46% in adults ages 40–64 years (Unutzer et al. 1998). Hirschfeld et al. (2003) sent the Mood Disorder Questionnaire (a validated screening instrument for bipolar I and II disorders) to 127,000 people. For the 85,258 responders, the overall screen rate for bipolar disorder was 3.4%, but for adults ages 65 years and older, the screen rate was 0.5%. Finally, Klap et al. (2003) conducted a telephone survey of 9,585 households and found a prevalence rate of 0.08% for adults ages 65 years and older compared with 1.17% for adults ages 30–64 years. Interestingly, each of these surveys suggested that the prevalence of bipolar disorder declines with age or in aging cohorts.