Giant aneurysms are the most serious issue of patients with Kawasaki disease (KD). To clarify risk factors for these giant aneurysms, we conducted a matched case-control study. Among the patients reported in nationwide surveys, 117 patients with giant aneurysms had an unequivocal new diagnosis and presented at the treatment center within 9 d of illness. We obtained clinical information on admission of about 69 patients (case) from the treatment centers. One control was selected for each case, an age- and sex-matched patient without coronary involvement, reported from the same treatment center at about the same time as the case, and we obtained the same clinical information about controls. Fourteen variables were analysed with a conditional logistic regression model: body temperature, hematocrit, hemoglobin, numbers of leukocyte and platelets, concentrations of serum albumin, globulin, total cholesterol, sodium, potassium and chloride, erythrocyte sedimentation rate, C-reactive protein and alanine aminotransferase activity. After adjustment for age, duration of illness before admission and use of intravenous gamma globulin therapy, C-reactive protein [odds ratio (OR)=1.142, 95% confidence interval (CI) 1.054-1.237], alanine aminotransferase activity (OR=1.008, 95% CI 1.002-1.014), serum sodium concentration (OR=0.877, 95% CI 0.770-0.999) and serum potassium concentration (OR=0.319, 95% CI 0.124-0.822) were significantly related to the risk for giant aneurysms. Further analyses with these four explanatory variables revealed that C-reactive protein (OR=1.159, 95% CI 1.022-1.315) and serum potassium concentration (OR=0.222, 95% CI 0.052-0.948) met the significant level. Thus, the values for serum C-reactive protein and potassium are independent risk factors for the development of the giant aneurysms of Kawasaki disease.