Serum creatine kinase (SCK) activity of the skeletal muscle (MM) form is sometimes moderately increased in acutely psychotic patients and may be massively increased as a result of muscle damage. The objective of this study was to characterize the SCK increases in patients treated with novel antipsychotic drugs. SCK activity and myoglobinuria, an index of gross muscle damage, were monitored at varying intervals in schizophrenic or schizoaffective patients treated with antipsychotic drugs. Possible causes of increases in SCK activity, such as trauma, excessive physical activity, exacerbation of psychosis, were assessed. Fifteen instances of massive increases in SCK activity were observed in 11 out of 121 patients (10%) treated with the following antipsychotic drugs: clozapine, loxapine, haloperidol, melperone, risperidone, or olanzapine. These increases in SCK activity were of the MM type and ranged from 1,206 to 177,363 IU/L (median, 9,600 IU/L). Thus, they were much larger than the increases usually found in acutely psychotic patients or patients with neuroleptic malignant syndrome (range, 500-3,000 IU/L). Only the patient with SCK activity of 177,363 IU/L had rhabdomyolysis as evidenced by myoglobinuria. The onset of the increases was from 5 days to 2 years after initiating treatment, and the increases lasted 4 to 28 days (median, 8 days). Flulike symptoms were present in two of the patients, but the others were asymptomatic. The increases were self-limiting in three cases, despite continuing treatment. Two of three cases rechallenged with the same drug again developed large increases in SCK activity within a week. It is unlikely these increases in SCK activity are related to acute psychosis, trauma, or the neuroleptic malignant syndrome. The increase in SCK activity may reflect the ability of the drugs to increase intermittently cell membrane permeability, especially in skeletal muscle, in some vulnerable subjects. A possible role of serotonin in this process is suggested by the pharmacology of most of the offending drugs. However, in some instances, the increases may have been unrelated to drug treatment. There was no evidence that these increases in SCK activity, despite their magnitude, compromised renal function. Routine monitoring of SCK activity of myoglobinuria during treatment with the antipsychotic drugs studied here is probably not necessary.