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Drooling (saliva loss) is a frequently reported symptom in patients with Parkinson's disease (PD), but an accurate estimate of the prevalence of drooling is lacking. The aim of this study was to systematically review the prevalence of drooling in published research papers. A systematic PubMed and CINAHL search was done, including studies published until January 2009. Eight studies were found, presenting prevalence rates of drooling based on responses of PD patients to questionnaires. The statistical heterogeneity was highly significant (P \ 0.0001), with prevalence rates ranging from 32 to 74%. The pooled prevalence estimate with random effect analysis was of 56% (95% CI 44-67) for PD patients and 14% (95% CI 3-25) for healthy controls; the pooled relative risk (RR) with random effect analysis was 5.5 (95% CI 2.1-14.4). All studies reported data of community dwelling idiopathic PD patients, with a mean age around 65 years and mild PD in 50-60% of the cases. Heterogeneity was mainly caused by differences in definition or frequency of drooling. The highest prevalence rates included nocturnal drooling where others noted only diurnal drooling. Analysis of the data of two studies showed that drooling is reported frequently by 22-26% of the patients. Prevalence rates were lower in milder PD patients. The summarized findings demonstrate that drooling can be present in half of all PD patients. In about a quarter of PD patients, drooling appears to be a frequently occurring problem. We recommend to report drooling in future studies with more detailed consideration of severity, frequency and nocturnal versus diurnal complaints.