Disability and progression in Parkinson's disease prior to levodopa treatment was investigated in a random group of 442 patients representing 91 per cent of all known cases with Parkinson's disease in a defined area. Almost half the patients were totally independent, 27 per cent needed help occasionally, and 25 per cent were dependent on other people. Ten per cent of the patients had only unilateral symptoms, 57 per cent had bilateral involvement with mild disability, 20 per cent had a moderately advanced disease, and 13 per cent were severely idsabled. A great variation was found in progression rates. In individual cases, however, the progression rate seemed to be relatively fixed. Concurrent arteriosclerosis was found to worsen the prognosis, whereas the beginning of the disease with tremor and the occurrence of prominent tremor promised a slower progression. Twenty-six patients with only unilateral symptoms 10 years after onset of the disease were found to correspond to postencephalitic patients in age structure, and in such cases a history of preceeding viral infection of the central nervous system was observed on four occasions.