Because the issue of how to empirically identify Mexican Americans in health-related research is still unresolved, the authors compared the performance of three indicators for identifying Mexican Americans across five distinct population subgroups: men and women in two age strata, and residents in low, middle, and high socioeconomic neighborhoods. Individual surname had the lowest sensitivity, specificity, and predictive values in the pooled population sample and varied the most widely on these parameters across population subgroups. Parental surnames, which are available on vital statistics and could easily be added to other health records used in secondary analyses, offered a significant improvement over individual surname in classifying persons as Mexican American. The San Antonio Heart Study (SAHS) algorithm, a nine-item indicator which uses parental surnames, birthplace of both parents, self-declared ethnic identity, and ethnic background of grandparents, had the highest sensitivity, specificity, and predictive values and varied the least on these parameters across different sex, age, and socioeconomic status population subgroups. The performance of all indicators was lower at the higher socioeconomic status levels. The findings suggest that it may be useful to use parental surnames as an indicator for Mexican-American ethnicity in research involving vital statistics and to add parental surnames to other health records frequently used in secondary analyses. Since the SAHS algorithm can be adapted for use with non-Mexican origin Hispanic subgroups, it may be a useful indicator for Mexican-American (or other Hispanic) ethnicity in survey research.