Acute encephalopathy was associated with the appearance of antibodies to human T-lymphotropic virus (HTLV-III) in two patients. A third patient showed seroconversion for anti-HTLV-III, but the temporal association was not established so precisely. The illness was characterised by a prodromal period of up to 2 weeks, characterised by pyrexia, general malaise, and changes of mood. The encephalopathy culminated in epileptiform seizures in two of the patients. Electroencephalographic changes were compatible with viral encephalitis, and cerebrospinal-fluid pleocytosis was minimal. Neurological signs and symptoms resolved quickly in all patients, and no residual central-nervous-system sequelae were apparent.