To test the hypothesis that acculturation influences MMPI performance of Native Americans, a 32-item instrument was developed to measure five components of acculturation. The MMPI-168 and the acculturation instrument were administered to 69 Rosebud Sioux. Significant elevations occurred on F and Scales 4, 6, 8, and 9. Intercorrelations among the acculturation subscales suggested a common dimension underlying social, language, and blood quantum, with values and education/occupation being relatively independent. The social, values, and language subscales were significantly related to validity scales L and F. A preponderance of significant correlations were obtained between values, language, and education/ occupation, and MMPI-168 clinical Scales 2, 4, 7, 8, and 0. These results suggest that caution be used in interpreting the MMPI profiles of Native Americans.