We describe an insert mutation in the prion protein (PrP) gene in a Japanese family line that encodes six octapeptide repeats. This is the second report to date of an inherited prion disease with a 144-base pair insertion, although the order of the repeat sequences differ from that reported for the disease in an English family line. The clinical features, like those of the English patients, were characterized by a slowly progressive generalized dementia with some neurological signs and cortical focal symptoms. Postmortem examination disclosed diffuse atrophy of cerebral gray matter and the cerebellar cortex; histologically, there were marked patchy and regional neuronal loss with astrocytosis in the frontal cortex, amygdala and hippocampus and PrP-immunoreactive plaques in the molecular layer of the cerebellum. These plaques were different from typical kuru plaques. The prion disease in the present Japanese family line is compared with that in the English family line.