The clinical characteristics of ALS in Finland were investigated. The study was based on the hospital records of 255 patients. No correlation was found between the disease and other previous illnesses. Muscular weakness was the most frequent complaint of the patients as their first symptom (149/255, i.e. 58 per cent). The duration of the disease was shortest (1.9 years) in male cases with the first symptoms in the bulbar area, and it was longest (3.4 years) in females with a spinal onset of the symptoms. The duration of the disease was in inverse relation to the age when it was contracted. Progressive muscular atrophy with fasciculations was considered the most important clinical sign of ALS. It was most frequently observed in the upper extremities including shoulders. Sensory disturbances were absent, sphincter abnormalities were reported in only three cases, bed sores did not exist, and there were no reports of impotence in the male patients. Two families with two familial cases in each were found, altogether two men and two women. Their disease did not differ from that of the rest of the series. The ALS patients soon became too disabled to work; 56 per cent bacame permanently disabled in less than 1 year's time, and only 9 per cent retained their working capacity for more than 2 years. The average length of stay at hospital was 32 days for ALS and 37 days for MS patients, but the later group was hospitalized 2-3 times more frequently than the former. Gastrostomy or tracheostomy and artificial respiration did not lenghten the lives of the patients.