The time course of evolution of clinical deficits has been a traditional guide to the nature of the aetiopathogenesis of neurological disease. We studied the influence of ageing and duration of disease on the natural history of idiopathic parkinsonism (IP). Two hundred and thirty-eight patients with IP were examined while off medication. Bradykinesia scores were analysed against patients' age and duration of disease by multiple regression. There was no significant interaction between the effects of age and of duration (P = 0.923). We conclude that age and duration of symptoms influence the natural history of IP additively and independently. Furthermore, the rate of neuronal death is more rapid in the earlier stages of evolution of the pathology; subsequently, the velocity of progression slows down to approach the rate of attrition produced by normal ageing. This time course has implications for possible models of pathogenesis.