The study reported here used data from the National Ambulatory Medical Care Survey (NAMCS) (4) to determine the prevalence of antipsychotic polypharmacy in ambulatory care. The NAMCS samples a random one-week period of a nationally representative group of visits to active physicians in nonfederal, office-based clinical practice. A total of 232,439 visits were sampled over the eight years, and data were weighted to produce national estimates. All visits during which at least one antipsychotic drug was prescribed, ordered, supplied, administered, or continued were included. Logistic regression was used to compute odds ratios (ORs).