What may be a new disease has been afflicting infants and young children in Japan since 1960. It is an acute, febrile, mucocutaneous condition accompanied by swelling of cervical lymph nodes (tentatively called mucocutaneous lymph node syndrome [MLNS]). It may be misdiagnosed as scarlet fever, the Stevens-Johnson syndrome, or infantile periarteritis nodosa. The disease is now known to be widely occurring all over Japan with an increasing incidence each year. More than 6,000 cases have been reported as of 1973. One to two percent of the patients reported have died suddenly of cardiac failure. All the autopsies showed infantile periarteritis nodosa-like arteritis accompanied by coronary thrombosis and aneurysm. Some of the surviving cases have been shown to have similar changes. These findings lead us to believe that this clinical picture is a new clinical entity. Recently, rickettsia-like bodies were found by electron microscopy in biopsy specimens from the skin and lymph nodes of the patients. The bodies were isolated by yolk sac culture and their pathogenicity is now under investigation.