The degree of viremia with human herpesvirus-6 was evaluated in 176 blood samples from 89 infants with exanthem subitum and viremia, and compared with the severity of clinical features and complications of the disease. Fever persisted for 3 to 4 days in 73% of infants and for more than 5 days in 22%, followed by a rubella- or measles-like rash. The viremia was observed between the first day of fever (day 0) and day 4 of the disease. The number of infected cells per 10 million mononuclear cells was 3.45 +/- 1.00 (log10, mean +/- SD) on days 0 to 2, 3.30 +/- 1.14 on day 3, and 3.09 +/- 2.05 on day 4 of the disease. The number of infected cells on days 3 to 4 in infants with a febrile period longer than 4 days and free virus in plasma was significantly greater than that in infants with a febrile period of less than 3 days and without free virus in plasma. The amount of virus in blood on days 0 to 2 did not relate to the duration of fever, and that on days 0 to 4 did not relate to the presence or absence of diarrhea, bulging fontanelle, or bronchopneumonia. These findings suggest that the magnitude of the virus replication in infants with exanthem subitum is reflected in the severity of the disease.