Sera from 248 children aged 30-179 weeks were tested retrospectively for human herpesvirus-6 (HHV-6) IgG antibody avidity. Twenty-five children presented with febrile fits, in one case with a rash, exanthem subitum resulting from primary HHV-6 infection was diagnosed at the time, but in the others HHV-6 was not considered in the differential diagnosis even though two patients had a rash. In fact, five of the 25 patients experienced primary infection as shown by low avidity HHV-6 antibody. Although nine other of these patients were seronegative in the acute phase of the illness, and unfortunately convalescent sera were not available to confirm primary HHV-6 infection, seronegativity was clearly a risk factor for febrile fits (P = 0.03, odds ratio 3.14 for all children; P = 0.002, odds ratio 7.20 for children aged 70-179 weeks), and primary HHV-6 infection was a very likely diagnosis. The remaining 11 children had high avidity antibody, so HHV-6 was excluded as a cause of their febrile fits. HHV-6 may frequently be a cause of febrile fits, often without the typical rash of exanthem subitum, and this diagnosis is often overlooked by clinicians.