To investigate the etiologic agent associated with Kawasaki disease (KD), we initially established a cocultivation system using concanavalin A (Con A)-stimulated lymphoblastoid cells obtained from each retrovirus-seronegative individual's peripheral blood mononuclear cells (MNCs) cocultivated with each of 1) 40 patients with KD, 2) 10 patients with other viral infection and skin rash, and 3) 10 age- and sex-matched normal controls. Five major findings suggested that virus-like particles with reverse transcriptase (RT) activity are associated with KD. First, RT activity appeared significantly higher on day 12 after the onset of fever in the KD patients than in those with other viral infections and normal controls (dTMP incorporation: 3,645 +/- 248 vs. 434 +/- 50 vs. 412 +/- 46 cpm, P < 0.0001). Second, the RT activity was not endogenous, because the Con A-stimulated lymphoblastoid cells were obtained from the individuals who were negative for retrovirus. Third, virus-like particles (80-100 nm in diameter) by electron microscopy were found in the concentrated pool supernatants of particulate fraction containing RT activity subjected to sucrose density gradient, obtained from KD patients. Fourth, the viral product, a 31.4 kilodalton molecule, was detected by SDS-PAGE after internal labelling (methionine-S35) and density gradient centrifugation. Fifth, using a "retrovirus universal pol gene region" as a primer and the RT-PCR method, a retrovirus-specific band was detected in the cocultivated supernatants obtained from four KD and one AIDS patients but not in patients with rubella or in healthy controls.(ABSTRACT TRUNCATED AT 250 WORDS)