OBJECTIVES: Clinicians treating schizophrenia face increasingly diverse populations, with different ethnic groups having different medication adherence patterns. The Texas Medicaid population with schizophrenia includes large numbers of patients from several ethnic groups, having broad use of the prevalent antipsychotic medications within each ethnic group. The purpose of this study was to assess the association between ethnicity and adherence among Texas Medicaid clients with schizophrenia, and to assess whether adherence within or across ethnic groups differed depending on which antipsychotic was used, after controlling for other factors. METHODS: Texas Medicaid claims were retrieved for persons, age 21 to 65, diagnosed with schizophrenia or schizoaffective disorder, initiating treatment with olanzapine (N=1875), risperidone (N=982), or haloperidol (N=726) between 1/1997 and 8/1998. For each of the three pairings of these medications, the association between ethnicity (African American, Mexican American, or Caucasian) or medication and days use of the medication in the year following initiation was assessed using multivariate linear regression. Covariates included other patient demographics, region, comorbid health conditions, and prior medication and heath care resource use. RESULTS: Overall mean adherence was 177 of 365 days (48.5%). African Americans and Mexican Americans were significantly less adherent than Caucasians in the haloperidol versus olanzapine and risperidone versus olanzapine comparisons (p <0.05 for each comparison). For patients of all ethnicities, olanzapine was associated with 19 more adherent days than risperidone and 56 more adherent days than haloperidol (p <0.001 for each pairwise comparison). CONCLUSIONS: When other factors were controlled for 1) ethnicity was a significant predictor of adherence following initiation on an antipsychotic medication and 2) patients of all ethnicities were most adherent when taking olanzapine, less adherent when taking risperidone, and least adherent when taking haloperidol.