Risperidone is an atypical antipsychotic commonly used for treatment of schizophrenia and other psychotic disorders. Although therapeutic drug monitoring is not routine for any of the atypical antipsychotics, serum antipsychotic concentrations are measured routinely to assess treatment nonadherence. In humans, risperidone is metabolized by cytochrome P450 2D6 to 9-hydroxyrisperidone; together these constitute the active moiety. Dose-proportional increases in serum concentrations have not been reported for the parent drug, but have been reported for 9-hydroxyrisperidone and the active moiety (i.e., the combined concentrations of risperidone and 9-hydroxyrisperidone). We describe a 34-year-old Caucasian man of Sicilian descent with a history of schizophrenia, disorganized type. He was suspected to be noncompliant with his risperidone therapy. Initially, active moiety risperidone concentrations increased linearly with prescribed dosage increases. However, with continued increases, active moiety concentrations adjusted downward and remained 17-36% below anticipated levels. We propose a method for estimating target active moiety concentrations of risperidone based on dosage-a method that may be used to guide clinicians in assessing nonadherence to risperidone treatment.