An aggregation of 14 cases of amyotrophic lateral sclerosis (ALS) was encountered in two families in Minnesota. Although the classical clinical features of ALS predominated, some members of one family showed, in addition, extrapyramidal signs, peripheral sensory impairment in the upper and lower limbs and mild mental fallout. Autosomal dominant inheritance with incomplete penetrance was the most likely mode of transmission. Pathological changes were the same as those seen in sporadic ALS although one patient also showed degeneration of the substantia nigra. These two families were compared to others in the literature and an effort was made to refine the classification of familial ALS.