We present the results of the first epidemiological study of ALS in Belgrade. The distribution of 58 newly discovered cases in a 7-year survey period (1985-1991) showed that the average annual age-adjusted incidence rate was 0.42 per 100,000 population (95% confidence interval, 0.18-0.83). The rate for males was 1.5 times higher than the rate for females. The greatest age-specific average incidence rate was observed in patients between 60 and 64 (3.66 per 100,000 population; 95% confidence interval, 2.17-5.78). The actual age-adjusted prevalence rate on December 31, 1991 was 1.07 per 100,000 (95% confidence interval, 0.71-1.71). The mean age at onset of the disease was 56.2 +/- 9.8 and it ranged from 24 to 74. We studied the natural course of the disease through the mean duration and cumulative probability of survival. The mean duration of the disease was 27.7 +/- 18.2 months. The cumulative probability of survival was 27% for the whole population in a 5-year interval. Elderly patients and those with bulbar signs at onset had a poorer prognosis. Patients under 49 at onset and those with the spinal form of the disease survived longer.