Serum HDL-cholesterol level and lipoprotein fraction were studied in 50 patients with Kawasaki disease (acute mucocutaneous lymph node syndrome), ranging in age from 4 months to 7 years, with an average of 2 years and 6 months. There were 31 males and 19 females. In 7 of these patients coronary arterial aneurysms were revealed by aortogram. In all 50 patients analysis were carried out periodically from the onset of the disease to the convalescent stage, and the following results were obtained: 1) Serum total cholesterol level showed mild change throughout the course of the disease. 2) Average HDL-cholesterol level was 29.6 +/- 11.0 mg/dl in the first week of the disease, 26.8 +/- 8.9 from 1 to 2 weeks and 47.9 +/- 12.6 after 6 weeks. There was significant difference between the HDL-cholesterol levels of first 2 weeks and after 4 weeks (p less than 0.01). 3) The recovery of HDL-cholesterol level was slow in both clinically severe patients and the patients with coronary aneurysms as compared with that in clinically mild patients (p less than 0.05). These findings suggest that there may be some relationships between the alteration of serum lipoprotein and the severity of systemic vasculitis in Kawasaki disease, especially which may play some role in clinical severity and the occurrence of coronary arterial lesions. However, further study is needed to clarify the cause of changes in serum HDL-cholesterol and lipoprotein.