In 107 Finnish patients with progressive myoclonus epilepsy (PME), belonging to 74 families, autosomal recessive inheritance was evident. The sex ratio was 48:51, the corrected proportion of affected sibs being 0.260. Of 68 marriages 15, or 22%, were consanguineous; several of the parents were related and the geographical distribution was of the uneven type typical of young, isolated populations in Finland. The incidence in Finland was estimated to exceed 1:20,000. The clinical picture in the Finnish PME patients was uniform, being identical with that of Unverricht's and Lundborg's patients, but clearly distinct from Lafora disease. The following classification of PME is proposed: (1) PME, Lafora type: onset of grand mal attacks and/or myoclonus around the 15th year of life; rapid and severe mental deterioration, often with psychotic symptoms; short survival; histological finding of Lafora bodies; autosomal recessive inheritance. (2) PME, Unverricht-Lundborg type: onset around the 10th year of life; severity variable, progressive invalidity from myoclonic features associated with mild mental symptoms, time of survival variable, "degenerative" histological changes; autosomal recessive inheritance. (3) Autosomal dominant or otherwise atypical cases of PME. The importance of accurate diagnosis is stressed.