Twenty-seven skin biopsies were obtained from the exanthemata of patients in the acute stage of the mucocutaneous lymph node syndrome (MCLS). The three vascular systems of different caliber size--the intrapapillary capillary loops (ICL), the superficial arteriolar or venular plexus (SAP, SVP) and the small subcutaneous vessels--were examined to investigate the characteristics of the vasculitis in MCLS and differentiate it from infantile polyarteritis nodosa (IPN). Significant papillary edema and dilatation of ICL, SAP and SVP were observed on the 4th day after the onset of the illness, and then gradually decreased. In the subcutaneous regions, vasculitis began with endothelial necrosis, and subendothelial edema and degenerative changes in the muscle cells followed. These changes in the small subcutaneous vessels were observed for a longer period than in the ICL, SAP and SVP. Moderate mononuclear cell infiltrations were observed. Both arteries and veins were affected.