OBJECTIVES: Given considerable variability in treatment response and medication side effects across individual patients using antipsychotics, customizing treatment to the needs of each individual is key to improving patient outcomes. This study examined the degree to which psychiatrists were diversified vs. concentrated in their choice of antipsychotic medication and identified factors associated with their prescribing. METHODS: Using 2011 data from Pennsylvania’s Medicaid we identified psychiatrists who regularly prescribed antipsychotics (defined as ≥10 nonelderly patients). Using prescriber ID we linked claims data, from which we obtained information on patient characteristics and psychiatrist prescribing behavior, to demographic information on psychiatrists from the AMA Masterfile, and to information on organizational affiliations from IMS Health’s HCOS ^TM database. We used three measures of antipsychotic prescribing concentration: Herfindahl index (HHI), share of most preferred ingredient, and number of ingredients. We used multiple membership linear mixed models to evaluate the degree of concentration for antipsychotic prescribing. Predictors included patient-, physician-, and organization-level factors. RESULTS: The analytic cohort included 764 psychiatrists treating 65,256 patients. Psychiatrists prescribed several ingredients (mean: 9); however, prescribing behavior was relatively concentrated (mean HHI: 2,603; share of most preferred ingredient: 37.8%), with wide variation across psychiatrists (range HHI: 1,088-7,270; share of most preferred ingredient: 16.4%-84.7%; number of ingredients: 2-17). Having a higher share of SSI-eligible patients, patients with serious mental illnesses, non-Hispanic whites, and older patients was associated with less concentrated prescribing although effects were relatively small (all p<.05). Female psychiatrists had more concentrated prescribing than that of their male counterparts (p<.10). Psychiatrists affiliated with behavioral health organizations had more diversified antipsychotic prescribing.  CONCLUSIONS: Antipsychotic prescribing behavior in a large state Medicaid program was relatively concentrated and varied substantially across psychiatrists. Some psychiatrists treating Medicaid enrollees with antipsychotics may be limited in their ability to tailor treatment to individual patient needs and preferences.