OBJECTIVES: To estimate the incidence of antipsychotic polypharmacy (APP) in the Texas Medicaid population. METHODS: Adults newly initiated on antipsychotics between July 1, 2006 and December 31, 2010 were followed for 365 days after the index antipsychotic claim. APP was defined as the concomitant use of two or more antipsychotics for at least 60 days without a gap in polypharmacy greater than 31 days. Monotherapy (MT) was defined as exposure to no more than one antipsychotic at a time during the one year after the index antipsychotic claim. Sensitivity analyses were conducted to determine how variations in the definition of APP impacted incidence.   RESULTS: Of the 23,232 patients meeting eligibility criteria, 5.4% (N=1,253) were on APP and 94.6% (N=21,979) on MT during the study period. Among those on APP, 70.1% used a combination of atypicals, 1.1% used a combination of typicals, 27.7% used a combination of a typical and an atypical, 0.8% used clozapine and an atypical, and 0.3% used clozapine and a typical. Among those on MT, 7% used typicals, 92.9% used atypicals, and 0.2% used clozapine. Patients with APP had a mean of 106.6±94.2 (median=86, interquartile range [IQR]=168) days prior to starting polypharmacy, a mean of 141.4±77.7 (median=118, IQR=108) days on polypharmacy, and a mean of 1.1±0.3 (median=1, IQR=0) APP episodes per patient. When the overlap period for antipsychotics was increased from 60 to 120 and 180 days, the incidence of APP fell from 5.4% to 2.2% and 1.3%, respectively. When the gap period was varied from 31 to 15 and 45 days, the incidence of APP changed from 5.4% to 4.6% and 6.9%, respectively. CONCLUSIONS: Although no clinical guidelines support APP, it was still seen for some patients in the study population. Future studies should be conducted to evaluate the costs and outcomes associated with this practice.