Kawasaki disease (KD) is an acute febrile illness of early childhood. Although the epidemiology of KD suggests an infectious agent, the cause still remains unknown. Intense immune activation during the acute disease has been well documented. Quantitative determination of soluble CD23 in serum can serve as an index of macrophage/monocyte or B-cell activation. To further characterize the immunological profile in KD, we investigated whether soluble CD23 levels in serum increase during the acute disease. In addition, we compared soluble CD23 levels in 33 patients with acute KD with levels in ten patients each with measles, rubella, infectious mononucleosis, and scarlet fever to determine if marked elevations in soluble CD23 were unique to acute KD. Patients with KD, rubella and infectious mononucleosis, but not patients with measles or scarlet fever, had increased soluble CD23 levels in serum during the acute stage, as compared to age-matched control subjects (P < 0.01). These data suggest infection with Epstein-Barr virus and rubella and acute KD are all characterized by B-cell and macrophage/monocyte activation.