The populations of T cells were studied in 46 patients with Kawasaki disease, separated into 2 groups: group I--11 patients with coronary aneurysms; and group II--35 patients with normal coronary arteries. Patients from both groups with early acute illness, before day 5, had a significant reduction in the population of OKT3+ (p less than 0.001), OKT4+ (p less than 0.02) and OKT8+ cells (p less than 0.002), but normal OKT4/OKT8 ratios compared with age-matched control subjects. These abnormal values quickly returned to normal levels during week 2 in patients with normal coronary arteries. In contrast, patients in whom coronary aneurysms developed within 3 weeks of the onset had an imbalance between OKT4 and OKT8 during week 2, characterized by a decrease in the number of OKT8+ cells and an increase in the number of OKT4+ cells, resulting in a high OKT4/OKT8 ratio (p less than 0.01). Three patients in whom large coronary aneurysms developed had ratios higher than 4.50. Follow-up analysis of T-cell subsets from individual patients with coronary aneurysms showed that the OKT4/OKT8 ratio during the acute stage was reduced during the convalescent stage (p less than 0.005). In contrast, the ratio in patients with normal coronary arteries was normal during the course of the illness. These observations suggest that an immune regulatory process operating in coronary aneurysm formation is present.