Elevation of skeletal muscle isoform of serum creatine kinase in heterozygous familial hypercholesterolaemia. Serum creatine kinase (CK) is an important adjunct in the diagnosis of myocardial infarction or in monitoring for the side-effects of drugs. We report elevation of CK in patients with untreated heterozygous familial hypercholesterolaemia (FH) with no evidence of myocardial infarction. Of a group of 30 patients, 60% had elevated CK values on one occasion. In 40% of patients elevated CK activity was observed on most occasions. This was not related to exercise or previous lipid-lowering therapy. These observations are of importance in view of the potential for misdiagnosis of myocardial infarction or premature withdrawal of lipid-lowering therapy. 