Two sets of monozygotic (MZ) twins with Kawasaki disease (KD) from two different families are reported. Twin set 1, previously healthy 71-day-old MZ twin girls were diagnosed with incomplete KD and pneumonia. The symptoms occurred at the same time in both girls. Both girls had ectasia of right coronary arteries. In twin set 2, the younger of 18-month-old MZ twin boys was diagnosed with KD and bronchitis. After 53 days, his elder brother was diagnosed with the same disease. The symptoms occurred at different time, but were almost identical. Neither boy displayed coronary artery changes. These findings support the hypothesis that genes susceptible to KD and coronary-artery lesions may exist in families. The different clinical characteristics among MZ twins from different families also suggest diverse and complex nature of KD.