We studied the onset of coronary artery aneurysm formation in 42 patients of Mucocutaneous lymph node syndrome (MCLS), the serial changes of T cell subsets, and serum immunoglobulin levels including lgG, lgA, lgM, in 13 patients among them during acute (before high dose gamma-globulin therapy ) and subacute phase (after gamma-globulin therapy) to evaluate the role of the immunoregulatory abnormalities in coronary artery aneurysm formation. The following results are obtainded : 1) The coronary artery aneurysm was found in 16 patients (38%). They were detected in between 6th and 13th day of illness with the mean of 8.8+/-2.1 day. 2) The total T lymphocyte (T1), helper T lymphocyte (T4), and helper/supperessor ratio (T4/T8) decreased during acute phase. 3) Following gamma-globulin therapy it showed a increase in T4,T8, and all immunoglobulines. Above results showed that the most of aneurysms developed during acute phase and suppressed, rather than enhanced, T cell function significantly contributed to aneurysm formation, whereas high dose gamma-globulin retarded or reversed this suppression which would be one of mechanism of beneficial effort of gamma-globulin in MCLS.