OBJECTIVE: The use of statins has been associated with risk for myotoxicity. Myopathy, defined as creatine kinase (CK) elevation greater than 10 times the upper limit of normal (ULN), occurs in 0.1 - 0.5% of statin users. This study describes elevated CK levels in an HMO population on cholesterol lowering therapy. METHODS: All subjects with a valid CK result during January 1 to December 31, 2001 were studied. Pharmacy claims data were linked to the laboratory results. A highly elevated result was defined as CK at least 10 ULN. RESULTS: A search of laboratory data identified 13,624 subjects with valid CK results. Most subjects had only one result (n = 10,301, 76%). Seventy-six subjects (0.6%) had a highly elevated CK result. Forty-one percent of all subjects (n = 5530) filled prescriptions for statins only, 400 (3%) filled prescriptions for non-statin cholesterol lowering therapy only, and 1473 (11%) filled prescriptions for both types of cholesterol lowering medications. Highly elevated CK results were found among 0.3% (n = 14) of statin only subjects, 0.5% (n = 2) of non-statin therapy only subjects and 0.6% (n = 9) of subjects with both medications. Of subjects with no cholesterol therapy, 0.8% (n = 53 of 6621) had a highly elevated CK result. CONCLUSIONS: We found a low rate of highly elevated CK results among those for whom results were available. The rate of highly elevated CK was higher among those with non-statin cholesterol therapy or combination therapy compared to those with statin therapy alone. Despite the low occurrence of highly elevated results, we found that a large percentage of those tested were statin users. The low incidence of highly elevated CK among monitored statin users raises questions regarding the most effective strategy for identifying patients at risk for myotoxicity.