To estimate the incidence of variant creatine kinase (CK)isoenzymes, 1,191 unselected patients were screened by means of CK:CK - MB ratio. When CK - MB was higher than 20% of the total CK in the immunoinhibition test, and a myocardial infarction could be excluded, a CK-isoenzyme electrophoresis was performed. We found macro-CK-BB-emia in 13 patients, but only three of them had elevated total CK activity. All patients but two were at the typical age of over 60 years; two males were 2 and 9 years old. The clinical diagnosis in macro-CK-BB-emia was predominantly cardiocerebrovascular insufficiency due to arteriosclerosis. In 16 other patients, the presence of an isoenzyme migrating cathodically to CK-MM was found. In this group, a malignant metastatic tumor was found in eight cases, the rest predominantly suffered from severe liver disease. Presence of both variant CK isoenzymes may lead to diagnostic or therapeutic errors due to the altered CK:CK-MB ratio. According to our studies, a variant CK isoenzyme occurs in 2.6% of hospitalized patients, but this will lead to problems diagnosing acute myocardial infarction only in the group with elevated total CK (0.42% of our patients).