Hemolysis can cause falsely elevated creatine kinase (CK) values when spectrophotometric methods of measurement are used. This apparent increase in CK is due to the red blood cell enzyme adenylate kinase. In an attempt to reduce this interference, most commercial CK kits employ adenosine monophosphate and/or diadenosine pentaphosphate as adenylate kinase inhibitors. To determine whether hemolyzed specimens should be accepted for testing, we measured the CK values of 26 serum samples, each with six different concentrations of added hemolysate. The results showed that hemolysis had an additive effect on CK, with an average increase in CK of approximately 10 U/L for every 1 g/L of hemoglobin. In most settings, this increase is not clinically significant. In the case of massive hemolysis, the hemoglobin concentration of the serum can be measured to correct the apparent CK value. The exclusion of hemolyzed specimens is unnecessary.