Exanthema subitum had been speculated to be a viral disease although its pathogen is unknown. Human herpesvirus 6 (HHV-6), first isolated in 1986, was proved by Yamanishi et al. to be the causal agent of exanthema subitum. To evaluate the role of HHV-6 as the causal agent in clinically diagnosed exanthema subitum, we tested for HHV-6 antibody in 57 infants with clinical exanthema subitum and exanthema subitum-like rash without fever. Of the 53 patients with clinical exanthema subitum 43 showed seroconversion or a significant rise in antibody titer to HHV-6, 7 were seropositive without significant rise and 3 remained seronegative. The clinical manifestations of these 43 infants with serologically confirmed HHV-6 infection were consistent with the classical characteristics of exanthema subitum. The 4 patients with atypical exanthema subitum showed significant rises in antibody titer. Our results therefore show that the majority of cases with typical clinical manifestations of exanthema subitum had HHV-6 infection. Most cases with HHV-6 infection had the typical clinical course of exanthema subitum, and a few cases might show an afebrile exanthema subitum-like rash.