We performed a population based survival analysis of all incident cases (220) of motor neuron disease (MND) in the province of Turin, Italy, during the period 1966-1980. 175 cases were diagnosed as amyotrophic lateral sclerosis (ALS), 43 as progressive muscular atrophy (PMA) and 2 as progressive bulbar palsy (PBP). The life-tables of MND, adjusted as to the "expected" mortality, showed a survival rate of 27.8% and 22.0% at 5 and 10 years, respectively. The course of PMA and ALS cases was different, with a 5-year survival rate of 66.8% and 17.7%, respectively. Nevertheless both life-tables showed a similar pattern with a rapidly fatal outcome in the first 3 years after diagnosis and a slower death rate in the following years. In each curve, the slopes suggested the presence of 2 subgroups with different prognosis. It is to be stressed that a percentage of PMA patients (25.7%) showed a rapidly fatal outcome and that a subgroup of ALS patients (26.6%) showed a relatively benign course. This might suggest a different individual susceptibility to the disease.