Background: Primary human herpesvirus-6 and -7 (HHV-6/-7) infections cause febrile illness sometimes complicated by convulsions and rarely encephalopathy. Aims: To explore the extent of such HHV-6 and -7 induced disease in young chi ldren. Methods: In a three year prospective study in Britain and Ireland, 205 ch ildren (2-35 months old) hospitalised with suspected encephalitis and/or severe illness with fever and convulsions were reported via the British Paediatric Sur veillance Unit network. Blood samples were tested for primary HHV-6 and -7 inf ections. Results: 26/156 (17%) of children aged 2-23 months had primary infect ion (11 HHV-6; 13 HHV-7; two with both viruses) coinciding with the acute illness; this was much higher than the about three cases expected by cha nce. All 26 were pyrexial; 25 had convulsions (18 status epilepticus), 11 requir ing ventilation. Median hospital stay was 7.5 days. For HHV-6 primary infection the median age was 53 weeks (range 42-94) and the distribution differed from t hat of uninfected children; for HHV-7, the median was 60 weeks (range 17-102) and the distribution did not differ for the uninfected. Fewer (5/15)-children w ith primary HHV-7 infection had previously been infected with HHV-6 than expec ted. Conclusions: Primary HHV-6 and HHV-7 infections accounted for a significa nt proportion of cases in those< 2 years old of severe illness with fever and co nvulsions requiring hospital admission; each virus contributed equally. Predispo sing factors are age for HHV-6 and no previous infection with HHV-6 for HHV-7 . Children with such neurological disease should be investigated for primary HHV -6/-7 infections, especially in rare cases coinciding by chance with immunisat ion to exclude misdiagnosis as vaccine reactions. Background: Primary human herpesvirus-6 and -7 (HHV-6 / -7) infections cause febrile illness sometimes complicated convulsions and rarely encephalopathy. Aims: To explore the extent of such HHV-6 and -7 induced disease in young chi ldren . Methods: In a three year prospective study in Britain and Ireland, 205 ch ildren (2-35 months old) hospitalized with suspected encephalitis and / or severe illness with fever and convulsions were reported via the British Pediatric Surillance Unit network. Blood samples were tested for primary HHV-6 and -7 inf ections. Results: 26/156 (17%) of children aged 2-23 months had primary infect ion (11 HHV-6; 13 HHV-7; with the acute illness; this was much higher than the about three cases expected by chance. All 26 were pyrexial; 25 had convulsions (18 status epilepticus), 11 requir ing ventilation. Median hospital stay was 7.5 days. For HHV-6 primary infection the median age was 53 weeks (range 42-94) and the distributi for HHV-7, the median was 60 weeks (range 17-102) and the distribution did not differ for the uninfected. Fewer (5/15) -children w ith primary HHV-7 infection had previously been infected with HHV-6 than expec ted. Conclusions: Primary HHV-6 and HHV-7 infections accounted for a significant proportion of cases in those <2 years old of severe illness with fever and co nvulsions requiring hospital admission; each virus contributed equally. Predispo sing factors are age for HHV-6 and no previous infection with HHV-6 for HHV-7. Children with such neurological disease should be investigated for primary HHV -6 / -7 infections, especially in rare cases coinciding by chance with immunisat ion to exclude misdiagnosis as vaccine reactions.