Elevated creatine kinase (CK) has frequently been described in patients on chronic dialysis, but little is known about its cause and distribution. We, therefore, measured CK in 105 patients on hemodialysis and continuous ambulatory peritoneal dialysis and compared it with biochemical, nutritional, and anthropometric data obtained at the same time. In the entire group, CK was 130.3 +/- (SEM) 15.0 IU/l. Thirty patients had elevated levels of enzyme (greater than 130 IU/l). Isoenzymes determined in patients with elevated CK levels were all more than 97% MM fraction. Men had significantly higher (p less than 0.001) CK values (166.0 +/- 25.8 IU/l) than women (82.4 +/- 9.0 IU/l). Blacks had higher CK values (158.8 +/- 21.7 IU/l; p less than 0.001) than whites (92.6 +/- 12.5 IU/l). Men and blacks had significantly higher weight and midarm muscle circumference than women and whites, respectively. A positive correlation was found between CK and lactic dehydrogenase (p less than 0.001) and between CK and midarm muscle circumference (p less than 0.05), and a negative correlation (p less than 0.01) was found with age. Predialysis and postdialysis CK was measured in 10 patients and did not rise. Three of the patients with elevated CK who have undergone successful renal transplantation showed normalization of CK levels. We conclude that CK is elevated in both hemodialysis and continuous ambulatory peritoneal dialysis patients, particularly in men and blacks, that CK levels are probably related to muscle mass, and that CK declines with advancing age. Although blacks have higher CK values as a whole, normalization of CK values after renal transplantation suggests a contributory role of renal dysfunction.