Introduction - We propose that the increase in TNF-α and IL-6 in the brain in idiopathic parkinsonism is in response to a peripheral immune/ inflammatory process, so ubiquitous as to be responsible for the resemblance between ageing and parkinsonism. Methods - Circulating cytokine was measured in 78 subjects with idiopathic parkinsonism and 140 without, aged 30 to 90 years, all obeying inclusion/exclusion criteria. Results - Serum TNF-α increased (P<0.0001) by 1.37 (95% CI 0.75, 2.00) %-y-1, IL-6 by 2.63 (1.75, 3.52) (P<0.0005). TNF-α appeared elevated in parkinsonians whose postural and psychomotor responses were abnormal, being suppressed where they were normal: trends which contrasted with those in controls (P=0.015 and 0.05, respectively). Parkinsonism appeared (P=0.08) to have an effect on IL-6, equivalent to that of >10 years of ageing (28(-3, 69)%), but was not immediately related to between-subject differences in performance. Conclusion -Ageing and pathogenetic insult may be confounded, age being a progression, not a risk, factor.