In five generations of the French M-E kindred, 11 members are now known to be or have been affected by a form of spongiform encephalopathy previously recorded as Gerstmann-Sträussler-Scheinker disease. Mean age at onset was 28 years (range 21-34 years). In six instances, these patients were hospitalized in psychiatric institutions with various diagnoses, the most frequent being mania or mania-like symptoms. Dementia occurred progressively after a lengthy course. Histological studies showed atrophy of the cerebellar molecular layer, which contained kuru and multicentric plaques labelled with anti-prion protein antibodies. Spongiosis was not prominent and remained largely limited to the periphery of plaques; it was more marked in the thalamus, where plaques were scarce. A 192 base pair (bp) insert (eight extra repeats of 24 bp) in the octapeptide coding region of the prion protein gene (PRNP) within a codon-129 methionine allele was found in four symptomatic subjects. Early age at onset, the prominence of psychiatric symptoms and the long course of the disease are noticeable clinical features in this family with an inherited prion disease due to a new insertional mutation in PRNP.