Amyotrophic lateral sclerosis is a progressive neurological disease of unknown etiology and fatal outcome. Patient management can be aided by careful assessment of prognostic factors. A prospective study of 158 patients was carried out to examine the prognostic significance of age and clinical form at onset. The overall 5-year survival rate was 14.7%. The higher the age was at first symptoms, the worse the prognosis. The bulbar and common forms had a worse prognosis than the pseudo-polyneuritic forms. After adjustment for age, the clinical form at onset remained a prognostic factor. In a multivariate analysis using the Cox model, these two factors remained independent despite the later onset of the bulbar forms. In view of the discrepancies between the different published studies, the evaluation of the survival of an individual patient is of doubtful value.