Racial classification of Asian subgroups is increasingly important for health statistics, given the growing Asian-American populations. This study reports the reliability of racial classification of Vietnamese in population-based cancer registry data from northern California. From the Greater Bay Area Cancer Registry, we selected 2240 persons diagnosed with cancer in 1989-1992 and whom the registry considered Vietnamese by birthplace and/or registry race and/or surname, or who were Southeast Asian or Chinese by race. One thousand ninety persons (49%) were interviewed. Sensitivity and predictive value positive, and cancer incidence rates, were calculated using different combinations of the classification factors (birthplace, registry race, and name). By registry-reported race alone, 74% of those the registry classified as Vietnamese agreed with this classification on interview, while 90% of those identifying themselves as Vietnamese were so classified. With classification based on 2 of 3 factors, 78% of those classified as Vietnamese agreed, and 91% of self-reported Vietnamese were correctly classified. Misclassification was associated with age, sex, year of immigration, education, and language use. Registry-based annual age-adjusted all-site cancer incidence rates per 100,000 for Vietnamese were 287.7 for males and 221.3 for females. Rates adjusted for self-reported ethnicity were 242.8 (male) and 213.7 (female). Registry classification of Vietnamese is currently problematic. Approximately 20% of cancer cases classified as Vietnamese are probably not Vietnamese. The higher incidence rates for Vietnamese in the United States than in Vietnam partly may reflect such classification error.