Eighteen patients with Kawasaki disease and suspected carditis (11 boys, 7 girls, mean age 18 months) in acute stages underwent Tc-99m HMPAO WBC imaging of the heart. Signs and symptoms subsided after conventional aspirin therapy and the intravenous injection of high-dose (400 mg/kg per day for 5 days) gamma-globulin treatments. Tc-99m HMPAO WBC imaging of the heart was arranged a second time to evaluate the effects of the treatments for carditis. The results showed that a significant difference existed in the severity of carditis before and after the treatments (P less than 0.001, by signed rank test), 39% (7/18) with significant improvement in severity of carditis, 50% (9/18) without definite change. However, 11% (2/18) became more severely ill after the treatments. Tc-99m HMPAO WBC imaging may be useful in detecting carditis in Kawasaki disease and in evaluating the effects of aspirin plus the newly recommended gamma-globulin for the treatment of carditis in Kawasaki disease.