Studies in adults have indicated a significant relationship between high serum creatine kinase levels on admission and acute psychosis. However, data on children are sparse. The flies of 183 hospitalized children and adolescents (93 boys, 90 girls) with severe psychiatric disorders were reviewed for serum creatine kinase activity on admission, psychomotor agitation, Clinical Global Impression Score, need for intramuscular injection, number of neuroleptic medications and presence of neuroleptic malignant syndrome. Serum creatine kinase levels > 201 IU/ml were considered abnormal. Boys had significantly higher creatine kinase activity than girls. Division of the cohort by diagnosis yielded significantly higher levels in those with schizophrenia, affective disorders and mental retardation. Higher levels were also associated with higher Clinical Global Impression score on admission, use of injections and physical restraint, and nonresponse to neuroleptic medication. There were no cases of neuroleptic malignant syndrome. This first large-scale investigation of serum creatine kinase activity in young psychiatric inpatients shows a significant association between high creatine kinase activity and acute psychosis, similar to that in adults. Furthermore, high creatine kinase levels on admission are predictive of the severity of the psychosis, but are not associated with neuroleptic malignant syndrome. Because psychotic adolescents with high admission creatine kinase levels tend to be nonresponders, clinicians should consider the early use of atypical antipsychotics in this Subgroup.