AIM: To evaluate the usefulness of the serum creatine kinase: aspartate aminotransferase (CK:AST) ratio in differentiating between myocardial and non-myocardial increases in serum creatine kinase activity in the elderly. METHODS: A three month prospective study of all patients admitted to an acute geriatric unit who were clinically assessed and investigated with electrocardiograms and measurement of CK, AST, and lactate dehydrogenase (LDH) activities on the first three days of admission. Excluding those with liver disease and alcohol misuse, patients with increased CK activity were then classified into four groups depending on whether they had fallen or had an acute myocardial infarction (AMI), or both. RESULTS: 270 patients were evaluated. CK activity was raised in 86 (31%) patients on any of the first three days of admission. Of these, 31 had fallen, 19 had an AMI, and five had both fallen and sustained an AMI. The CK:AST ratio, on all days, was higher (p < 0.05) in those who had fallen. On the second and third days, the CK:AST ratio was higher (p < 0.01) in those patients who did not have an AMI. The overlap, however, between these groups was large. CONCLUSIONS: These results did not allow discrimination to be made between myocardial and skeletal sources for increased CK activity. The CK:AST ratio is, therefore, of limited use when applied to the diagnosis of AMI in elderly patients. Clinical evaluation rather than the pattern of enzyme change is more likely to determine the cause of increased CK activity.