Measurement of creatine phosphokinase (CPK) in sera from patients with pneumonia or pulmonary emboli confirms our earlier finding of increased serum enzyme activity in some such patients. The abnormalities found were more striking in patients with pneumonia than in those with pulmonary emboli. Eleven samples of lung tissue obtained at autopsy were studied; all showed modest levels of CPK activity. Electrophoresis revealed an anodic and intermediate isoenzyme in each sample. One of the 11 also showed a cathodic isoenzyme similar to that seen in muscle tissue. These observations reemphasize that elevated serum CPK activity does not have absolute diagnostic specificity.