Complement activation, a key component of neuroinflammation, has been reported in both Parkinson's disease (PD) and Alzheimer's disease (AD). However, it is unclear whether complement activation and neuroinflammation in general are distinctly different from each another in major neurodegenerative disorders. In the present study, cerebrospinal fluid complement 3 (C3) and factor H (FH) were measured and evaluated together with amyloid-beta(42) (A beta(42)), which in recent investigations was decreased in patients with PD, in particular those with cognitive impairment. The study included 345 participants: 126 patients with PD at various stages with or without cognitive impairment, 50 with AD, and 32 with multiple-system atrophy, and 137 healthy control individuals. In addition to changes in A beta(42) concentrations, there were clear differences in the patterns of complement profiles among neurodegenerative disorders. The C3/FH ratio demonstrated high sensitivity and specificity in differentiating patients with multiple-system atrophy from those with AD or PD and control individuals. In addition, the C3/A beta(42) and FH/A beta(42) ratios not only correlated with PD severity approximated using the Unified Parkinson's Disease Rating Scale but also with the presence of cognitive impairment or dementia in PD. Both C3 and FH correlated with the severity of impairment in AD as indicated using Mini-Mental State Examination scores. (Am J Pathol 2011, 178:1509-1516; DOI: 10.1016/j.ajpath.2011.01.006)