PURPOSE: To search for evidence of B cell activation and superantigen involvement in Kawasaki disease. METHODS: Peripheral lymphocytes were first isolated from Kawasaki disease patients in the acute and subacute phases. The T cell and B cell distributions were analyzed, cDNA was generated from the total RNA extracts, and PCR amplification of the cDNA for each immunoglobulin heavy chain family was performed to determine the presence of VH family-specific oligoclonal expansion. CDR3 size analysis was then conducted by two-stage PCR. RESULTS: The percentage of B cells increased significantly(P<0.05) in both the acute and subacute phases. Random utilization of diverse VH family genes was observed in the acute phase, and no family-specific expansion was detected. In 13 out of 15 Kawasaki disease patients, an increase in B cells expressing the VH3 family was seen during the acute phase, making it the most frequently utilized family. Analysis of B cell clonal expansion showed the VH6 family clone of 9 amino acids to be the most common clone, observed in 5 out of 15 Kawasaki disease patients. Analysis of the CDR3 size profile in two patients showed that in the acute phase various prominent bands appeared, some disappearing in the subacute phase, while other newly developed bands appeared. CONCLUSION: VH family-specific B cell expansion was not detected, and clonal expansion of B cells was observed, suggesting that Kawasaki disease may be caused by a conventional antigen, and the antigenic stimulation seen during the acute phase seems to be continuous, resuming after clinical resolution.