Objective: This study evaluated antipsychotic use in Medicaid beneficiaries with a schizophrenic disorder and identified factors associated with poor adherence. Methods: This study involved a retrospective cohort analysis of non-dual Florida Medicaid recipients who had a medical claim indicating a schizophrenic disorder (ICD-9-CM 295.XX) and received an antipsychotic (APS) medication between July 1, 2004 and June 30, 2005. Patients were followed for one year after the first APS prescription. Adherence was measured using the Medication Possession Ratio (MPR: defined as unduplicated ambulatory treatment days divided by the number of ambulatory days in the period), medication persistence (days between the first and last antipsychotic in the follow-up period), and number of untreated days. Logistic regression models were used to identify predictors of poor adherence (MPR<0.80). Results: A total of 8828 patients met inclusion criteria. Mean (±SD) age was 42.3 (±13.7) years, 49% were female, and 36.8% were white. Approximately 18% and 39% had pre-existing diagnoses of substance abuse or other psychiatric conditions, respectively. Mean (±SD) MPR was 0.72 (±0.3). The mean number of untreated days was 47.4 (±60.8), and mean persistence was 311.9 (±102.5) days. Approximately 57% of patients had MPR values between 0.8 and 1. Logistic regression indicated that younger patients (<18 years), females, nonwhites, those with a substance abuse diagnosis or who received antidepressants, and those newly starting APS therapy were significantly more likely to be poorly adherent, while those treated with atypical or injectable antipsychotics (vs. conventional orals) were less likely to be poorly adherent. Conclusion: Several patient characteristics are predictive of poor adherence to APS therapy. Study findings may be informative to health plan administrators interested in identifying patients at risk for medication non-adherence.