The aim of this study was to investigate the correlation between clinical diagnosis of exanthema subitum and seroconversion of herpes virus type 6 (HHV-6). 90 children with fever of unknown origin were investigated by 11 pediatricians in the Canton of Zürich, Switzerland, between October 1989 and June 1990. Antibodies against HHV-6 were determined at the first consultation and 2-3 weeks later. History and clinical findings were investigated and the clinical probability of the diagnosis exanthema subitum was estimated. 82 of the 90 children could be evaluated. 33 of them already had HHV-6 antibodies in their first blood sample although their history was negative concerning exanthema subitum. In 24 children exanthema subitum was proven by seroconversion. The clinical diagnosis was much more reliable at the second consultation compared to the first one. The clinical course of the disease was highly variable. Duration of fever differed widely and in some children the exanthema was atypical or even absent.