OBJECTIVE To evaluate the effectiveness of intravenous immunoglobulin (IVIG) treatment of Kawasaki disease ≥10 days after illness onset. METHODS We selected patients initially treated with IVIG on days 11 to 20 in the database of the 20th nationwide survey in Japan. We then selected pair-matched control subjects of the same gender and age, who were initially treated with IVIG on days 4 to 8 with the same dose at the same institutions. We compared the proportions of additional treatments and coronary artery lesions (CALs) between the groups. We also compared fractional changes in various laboratory data before and after IVIG. Fractional change was defined as follows: (Y - X)/X, in which X represents the data before treatment and Y the data after treatment. RESULTS One hundred fifty patients (75 pairs) were studied. The proportion of patients who received additional treatments among those given initial IVIG after days 10 was slightly lower than those treated earlier (12% vs 16%). The fractional changes in the white blood cell count, % neutrophils, and C-reactive protein were similar. Among all patients, the proportions of CALs during the convalescent phase were significantly higher in the late than in the early group (27% vs 1%). Among patients who had not developed CALs before initial treatment, the proportions with CALs during the acute phase were similar (8% vs 8%). CONCLUSIONS IVIG treatment ≥10 days after illness onset achieves resolution of inflammation but was found to be insufficient for preventing CALs.