The incidence and significance of elevated serum levels of creatine phosphokinase (CPK) in febrile diseases were studied prospectively in all patients admitted with fever to a department of medicine during 1 year. High serum CPK levels were detected in 70 (28%) of 247 febrile patients but in only six (6%) of 105 afebrile control patients (P = .0001). Elevated CPK levels were not related to any specific diagnosis. Logistic regression analysis identified five factors that correlated both significantly and independently with elevation of CPK values: increased blood urea nitrogen level, low serum phosphate level, a stuporous or comatose state, tremor, and muscle tenderness. Myoglobinuria, detected in 14 patients, was predictive of a fatal outcome, but a high CPK level by itself was not an independent correlate of mortality. In summary, CPK elevation is not uncommon in febrile diseases, but because it does not reflect a specific etiology it does not necessarily indicate that an extensive diagnostic work-up is required.