We measured with a radioimmunoassay the concentrations of carbonic anhydrase III (CA-III, EC 4.2.1.1) in sera from 68 patients with muscular dystrophy, 10 carriers of Duchenne muscular dystrophy (DMD), and 63 patients with other neurological disorders. The values obtained were compared with those for creatine kinase (CK, EC 2.7.3.2). Serum CA-III was strikingly increased in patients with DMD (mean, 274.4 micrograms/L) and congenital (Fukuyama-type) (182.8 micrograms/L) and limb-girdle (203.7 micrograms/L) dystrophies and positively correlated with the activities of CK in patients with DMD. CA-III concentration decreased with the subjects' age and the severity of the disease, similar to the tendency observed between age or severity and the concentration of CK. We found moderately increased CA-III in patients with polymyositis, myotonic dystrophy, amyotrophic lateral sclerosis, spinal progressive muscular atrophy, or Kugelberg-Welander disease and in carriers of DMD.