A 14-month-old girl presented after 3 days of fever, floppiness, and diffuse urticarial exanthem. She developed encephalitis and carditis and 1 week later, intractable seizures. Initial CT and MRI showed no changes in the brain parenchyma. On days 14 and 34 after the onset of symptoms, a human herpesvirus-6 (HHV-6) genome in cerebrospinal fluid was identified by polymerase chain reaction (PCR). Convulsions became more frequent and 11 weeks from the onset, they changed to typical infantile spasms with hypsarrhythmic electroencephalogram. She gradually lost her social contact and ability to walk and sit. Eleven months after the primary infection, a repeated MRI of the brain revealed a cystic tumour of 2 cm in diameter near the vermis. The tumour was surgically removed, and shown to be a pilocytic astrocytoma on histopathological examination. HHV-6 DNA was detected by PCR in new tumour tissue. This is the first reported case of HHV-6 encephalitis associated with carditis, infantile spasms, and a subsequent brain tumour containing the HHV-6 genome.