Objective: To estimate prevalence rates of antipsychotic use in children and adolescents from 1996 to 2001 in three state Medicaid programs (midwestern [MM], southern [SM], and western [WM]) and one private managed care organization (MCO). Method: Prescription claims were used to evaluate antipsychotic prevalence, defined as the number of children and adolescents up to the age of 19 years with at least one prescription claim for an antipsychotic per 1,000 enrolled youths. Results: From 1996 to 2001, the prevalence of total antipsychotic use increased in each program (MM: 4.7 to 14.3 per 1,000; SM: 6.3 to 15.5; WM: 4.5 to 6.9; and MCO: 1.5 to 3.4). Typical antipsychotic use decreased (MM: 3.7 to 2.0 per 1,000; SM: 4.6 to 1.5; WM: 4.4 to 1.3; and MCO: 1.2 to 0.9), while atypical antipsychotic use dramatically increased (MM: 1.4 to 13.1 per 1,000; SM: 2.5 to 14.9; WM: 0.3 to 6.2; and MCO: 0.4 to 2.7). Conclusions: The increased prevalence of antipsychotic use in children and adolescents from 1996 to 2001 was attributed to increased use of atypical antipsychotics. Given the limited data with atypical antipsychotics in youths, this emphasizes the need for additional studies of these agents and other treatment modalities in this population. J. Am. Acad. Child Adolesc. Psychiatry, 2005;44(6):548-556. Key Words: antipsychotics, pharmacoepidemiology.