The prevalence of diabetes among 29,000 American Indians living on or near 10 reservations in the Pacific Northwest was established from clinical data sources. The age- and sex-adjusted prevalence rate in 1987 was three times higher [95% confidence interval (CI) 2.8-3.1] than the comparable United States rate in 1980. The age-adjusted prevalence for women was 1.3 times that for men (95% CI 1.2-1.4). The prevalence varied by culture area. Reservation communities whose principal tribe came from the Great Basin culture area had 3.6 times more diabetes than found in the U.S. as a whole. Reservation communities whose principal tribe was from the Plateau culture had 3.0 times more diabetes than found in the U.S., and those from the Northwest Coast culture had 1.9 times more diabetes than found in the U.S. The reasons for these variations are unknown and warrant investigation.