It has been reported that Kawasaki syndrome is accompanied with mitral regurgitation or aortic regurgitation in some cases. To elucidate the incidence and the natural history of valvular heart disease in Kawasaki syndrome, we analyzed the patients who were detected to have a new heart murmur after the onset of Kawasaki syndrome. From 1973 to 1988, we have experienced 13 cases with valvular heart disease in 1215 cases of Kawasaki syndrome, 12 cases with mitral regurgitation (1.0%) and one with aortic regurgitation (0.1%). Valvular lesions were confirmed by angiography or pulsed Doppler echocardiography. The age at onset of Kawasaki syndrome, the duration of fever, the maximum erythrocyte sedimentation rate, and the incidence of coronary artery lesions in these cases were compared with the same variables in 30 cases of without valvular lesion in Kawasaki syndrome. There were no statistical difference between the cases with valvular heart disease and without valvular heart disease about the age of onset (mean +/- SD 10.2 +/- 12.7 months vs 20.8 +/- 18.4 months; N.S.) and the maximum erythrocyte sedimentation rate (87.7 +/- 29.0 mm/h vs 87.2 +/- 35.6 mm/h; N.S.). Whereas the duration of fever in cases of valvular heart disease was more extended than those without valvular heart disease (20.3 +/- 8.1 days vs 10.3 +/- 4.3 days; p less than 0.001), and the incidence of coronary artery lesions in the cases of valvular heart disease was significantly higher than those without valvular heart disease (12/13 cases vs 7/30 cases; p less than 0.001), thus suggesting that the cases of valvular heart disease were subject to a severe case of Kawasaki syndrome. All valvular heart disease appeared within 1 month after the onset of Kawasaki syndrome, except in one case whose heart murmur was noticed 5 years after the onset. The heart murmur disappeared within 2 months after the onset of valvular heart disease in 5 cases, however in another 7 cases, the heart murmur persisted more than 2 years (mean; 5.3 years to date) or one has died of acute congestive heart failure due to mitral regurgitation. All cases with persistent valvular disease revealed mitral or aortic valve prolapse. Our data suggest that the cause of valvular heart disease might be different by the time of onset and duration of valvular heart disease.(ABSTRACT TRUNCATED AT 400 WORDS)