Amyotrophic lateral sclerosis (ALS) was diagnosed in 36 patients in Middle-Finland Central Hospital District during 1976-1981. The annual incidence of ALS was 2.4 per 100,000 population and the prevalence rate was 6.4 per 100,000 population. The age-specific incidences of ALS were similar for men and women with a maximum of 14/100,000/year in the age group 60-69 years. The initial symptoms originated in 37% of the patients from bulbar and in 63% from spinal levels. Bulbar onset was more common in patients aged 60 years or more compared with younger patients. Patients with bulbar onset had a significantly poorer prognosis than those with spinal onset, which explained the poorer prognosis of older patients. 4 matched controls were chosen for each ALS patient from the files of the Central Hospital. There was no difference between the patients and the controls with respect to previous injuries, surgical operations, malignant neoplasms, or exposure to domestic animals. An earlier observation that evacuees from Karelia ceded to USSR after World War II should have a prevalence twice that of the remaining population was not substantiated.