The contribution of serum creatine kinase (CK) levels to the diagnosis of acute myocardial infarction (AMI) in an emergency room was studied in 252 patients presenting with chest pain. Thirty percent were ultimately diagnosed as having AMI. The electrocardiogram (ECG) identified 66% of patients with AMI who were evaluated within 4 h of onset of symptoms; while CK serum levels were elevated in only 9%. Among patients evaluated more than 4 h after the onset of symptoms, the ECG was helpful in diagnosing AMI in only 36.6%, while serum CK levels were high in 63.4%. CK testing added significantly to the diagnosis of AMI in patients already studied by ECG. We suggest that determination of serum CK levels in the emergency room is of value in the evaluation of patients complaining of chest pain 4 or more hours after the onset of symptoms.