This study was a 13-month prospective, descriptive case series of risperidone overdose reported by telephone to a regional poison control center (PCC) serving Philadelphia, PA. Patients were seen in local Philadelphia-area emergency departments. The variables examined were medical history, therapeutic use of risperidone, time postingestion, reported coingestants, clinical findings, decontamination and treatment, electrocardiograph results, laboratory data, standard toxicologic screen results, and length of time in hospital. Thirty-one patients (29 adult/adolescent, 2 pediatric) with reported risperidone overdose were identified. Risperidone was the sole ingestant in 15 cases (1 mg to 180 mg). The major observed effects in this group included lethargy (7), spasm/dystonia (3), hypotension (2), tachycardia (6), and dysrhythmia (1). Sixteen cases involved coingestants, including benzodiazepines, selective serotonin reuptake inhibitors, ethanol, tricyclic antidepressants, lithium, anticonvulsants, diphenhydramine, ibuprofen, and anticholinergic agents. Major effects in these patients included lethargy (10), coma (1), seizure (1), tachycardia (7), bradycardia (1), hypotension (4), and a syndrome of muscle spasms, diaphoresis, and fever. Treatment provided for patients in this study included antiarrhythmics (1), diphenhydramine (2), anticonvulsant (1), vasopressor agent (1), endotracheal intubation/assisted ventilation (5), and supportive care. One patient who coingested imipramine died of medical complications. In the remaining patients, symptoms resolved with 24 hours in the majority, with all patients asymptomatic at 72 hours postingestion. These data show that risperidone toxicity manifests primarily as mild central nervous system effects and reversible neuromuscular and cardiovascular effects.