Serum carbonic anhydrase III (CAIII) levels were determined by means of an enzyme immunoassay method and were compared with serum creatine kinase (CK) and muscle-specific enolase (MSE) levels in 33 patients with myotonic dystrophy. Serum CAIII levels were elevated in all 33 patients, whereas serum CK and MSE levels were elevated in 12 and 10 patients, respectively. Serum CAIII levels showed a good correlation with CK levels, but a poor one with MSE levels. There was no obvious correlation between the serum CAIII level and the duration of illness or the age of the patient. These results suggest that serum CAIII is probably a more sensitive marker than CK and MSE in myotonic dystrophy and may also reflect the type 1 fiber abnormality more predominantly observed in myotonic dystrophy.