The authors identified 298 diagnoses of moyamoya in California and Washington from hospital discharge databases during the period 1987 to 1998. The incidence was 0.086/100,000 persons. The ethnicity-specific incidence rate ratios compared to whites were 4.6 (95% CI: 3.4 to 6.3) for Asian Americans, 2.2 (95% CI: 1.3 to 2.4) for African Americans, and 0.5 (95% CI: 0.3 to 0.8) for Hispanics. The incidence of moyamoya in Washington and California was lower than reported in Japan, but the rate among US Asians is similar.