Boxing is often believed to be a frequent cause for parkinsonism caused by chronic repetitive head injury, with Muhammad Ali frequently cited as an example. The purpose of this study is to determine the prevalence of Parkinson's disease (PD) in retired Thai traditional boxers. Two standardized screening questionnaires were sent to all registered Thai traditional boxers. Subjects who screened positive for parkinsonism were invited for clinical examinations by two independent neurologists. Among 704 boxers (70%) who completed the questionnaires, 8 boxers (1.14%) had parkinsonism: 5 with PD, 1 with progressive supranuclear palsy and 2 with vascular parkinsonism. Boxers with PD were found to have an older mean age than those without PD (P = 0.003). The analysis of probable risk factors disclosed an association between the number of professional bouts (>100 times) and PD (P = 0.01). The crude prevalence of PD in Thai boxers was 0.71% (95% CI: 0.09-1.33), with a significant increase with age. The prevalence rate of PD in those aged 50 and above was 0.17% (95% CI: 0.15-0.20), age-adjusted to the USA 1970 census, which is comparable to that of the general populations. The analysis determined that the number of professional bouts is a risk factor among these boxers, supporting the notion that repetitive head trauma may pose an additional risk to certain individuals who are already susceptible to PD. © 2010 Movement Disorder Society.