A retrospective analysis done on all available charts of each patient admitted to a psychiatric inpatient service during a one-year period provided 144 cases. Thirty-nine of these (27%) showed elevations of serum creatine phosphokinase (CPK) activity, but 16 of these 39 elevations (41% of total CPK elevations) occurred in patients who had received intramuscular injections and, consequently, were excluded from this study. Of the remaining 128 patients, 35 were diagnosed "psychotic" by mental status criteria. The dimension of "acuteness" of psychosis could not be precisely assessed, but 19 of the psychotic patients (54%) had elevations of serum CPK activity. Of the 93 patients determined to be "nonpsychotic" by mental status criteria, only four (4%) had CPK elevations. These elevations were unexplained. The probability of this distribution occurring on a chance basis is < .001. The elevations in the nonpsychotic group were for the most part unexplained. In contrast to some earlier reports by others, a significant portion (74%) of the patients with elevated CPK values also had elevated serum glutamic oxaloacetic transaminase (SGOT) levels.