SummaryOn a rearing farm with 96,000 birds, 10,000 three and four days old chicks died with nervous symptoms.A virus was isolated from the brains and identified as an Aujeszky's disease virus. The isolate was very pathogenic for chickens up to about 7 days of age, causing mortality after parenteral injection (intracerebral, intraperitoneal, intramuscular) but not after oral, eye drop or spray application.An Aujeszky vaccine virus, made apathogenic by passages in chicken cells for use in swine, had the same pathogenic properties for chicks. The isolated Aujeszky's disease virus is regarded as the agent responsible for the death of the 10,000 chicks on the farm. This virus most likely had been injected in just hatched chicks instead of or together with the Marek vaccine virus.In addition to meningitis, edema, neuronophagia and cuffing of blood vessels with mononuclear cells, haemorrhages were observed in thin sections of brain and spinal cord. After injection of isolate and vaccine virus in the leg muscle intranuclear inclusion bodies were observed in the ganglion cells in the spinal cord. Inclusion bodies have not been described before in pathological conditions of the nervous tissue of chickens.