Two studies were conducted to examine the relationship of acculturation to neuropsychological test performance among (1) medically healthy, neurologically normal African Americans (N = 170); and (2) HIV positive (HIV+) subgroups of African Americans and Whites (Ns = 20) matched on age, education, sex, and HIV disease stage. Acculturation was measured through self report for all participants, and linguistic behavior (Black English use) was assessed in a subset of medically healthy individuals (N = 25). After controlling for the effects of age, education, and sex, medically healthy African Americans who reported less acculturation obtained lower scores on the WAIS–R Information subtest and the Boston Naming Test than did more acculturated individuals. Black English use was associated with poor performance on Trails B and the WAIS–R Information subtest. HIV+ African Americans scored significantly lower than their HIV+ White counterparts on the Category Test, Trails B, WAIS–R Block Design and Vocabulary subtests, and the learning components of the Story and Figure Memory Tests. However, after accounting for acculturation, ethnic group differences on all measures but Story Learning became nonsignificant. These results suggest that there are cultural differences within ethnic groups that relate to neuropsychological test performance, and that accounting for acculturation may improve the diagnostic accuracy of certain neuropsychological tests. (JINS, 1998, 4, 291–302.)