Over the past 25 years, the clinical course of Kawasaki disease has been defined, the prevalence and nature of the cardiovascular effects widely understood, and pathological changes in the most severe cases well described. However, the aetiology and pathogenesis of this puzzling disease have remained unclear, thus specific therapy is not yet available. Because of some close clinical similarities between this disease and streptococcal scarlet fever, particular attention has been paid to the possible role of Streptococcus pyogenes as an aetiological agent in this illness. Until now, however, group A beta-haemolytic streptococci have never been consistently isolated from any patients; in addition, the titre of anti-streptolysin 0 is not raised, and lack of response to antibiotics is a feature of this disease. Our long series of investigations over more than 10 years, which will be covered in the present review, were performed in an attempt at elucidating causative agent(s) of Kawasaki disease. This has led to our firm belief in the probable role of S. pyogenes in the pathogenesis of this disease, despite the lack of fulfillment of Koch's postulates, on the basis of the following findings. Patients with Kawasaki disease recovering from the acute, febrile phase of the illness exhibited an exaggerated cell-mediated reactivity, as measured by the macrophage migration inhibition test, to group A beta-haemolytic streptococci, their pyrogenic exotoxin and streptolysin 0 as well as to several mammalian muscle cell extracts which are allegedly related antigenically to the cell wall and/or cytoplasmic membrane of S. pyogenes. Protoplast-like "spherical bodies" varying in diameter from 0.5 to 1.5 microns, and devoid of cell walls, were detected in the buffy coats of peripheral blood from patients with this disease, and stained distinctly by immuno-electron microscopy using, as a primary antibody, a rabbit antiserum to S. pyogenes- derived protoplasts, and followed by absorption with protoplasts from Staphylococcus aureus and Escherichia coli. Newborn mice infected with S. pyogenes having no capacity to confer cell-mediated immunity even in adult murine hosts, and reinfected 4-6 weeks later with another strain of the same species of bacteria which is able to elicit cellular immunity, showed a lack of humoral response to streptococcal antigens, leaving intact cell-mediated immunity. Such a biased immunological characteristic is an exact counterpart of that of Kawasaki disease patients.(ABSTRACT TRUNCATED AT 400 WORDS)