Serum adenosine deaminase (ADA) activity and peripheral lymphocyte subsets of patients with myasthenia gravis (MG) were simultaneously measured. The ADA activity in MG (n = 30) was significantly higher as compared with normal control (n = 150) and multiple sclerosis (n = 12) (P less than 0.05). The ADA activity of generalized MG was higher than that of ocular MG, while a significant elevation of ADA activity was observed in grade IIB as compared with grade I of Osserman's classification (P less than 0.05). A trend of high ADA activity was demonstrated in those whose disease had advanced to a severe degree associated with unstable clinical features (P less than 0.05). In addition, there was a significant elevation of ADA activity in patients who disclosed positive anti-Ach-receptor-antibody as compared with negative one (P less than 0.05). There was no specific trend among the proportions of the subsets of peripheral lymphocytes which could reflect the severity of MG, however, the proportion of OK Ia1+ tended to be higher with advancing the grade of MG. Interestingly enough, a close correlation was found between the ADA activity and the proportion of OK Ia1+ cells (P less than 0.05). From the above results, it was concluded that high ADA may be responsible for the pathophysiology of MG through the alteration of peripheral lymphocyte function.