Clubroot disease caused by Plasmodiophora brassicae is an important disease of cruciferous plants. Although the pathogen is widespread and has been reported to cause high yield losses, the impact on winter oilseed rape (OSR) has not been experimentally verified. To quantify the risk potential of P. brassicae, we conducted two closely linked experiments. A semicontrolled experiment used artificial soil infestation at inoculum densities between 106 and 108 spores liter-1 of soil to detect the impact on seed yield and yield components of a susceptible and resistant OSR cultivar. A greenhouse experiment was implemented using the soil of the semicontrolled experiment after cropping the two cultivars to quantify the influence of cultivar resistance on soil inoculum. According to cumulative link mixed models, disease rating was positively correlated with the amount of inoculum. Linear regression analyses revealed a negative correlation between seed yield and inoculum density. Yield losses of 60% already appeared at the lowest inoculum density. Plant losses and reduced seed per pod were accountable for yield losses. Although the resistant cultivar showed clubroot symptoms, seed yield was not affected by the pathogen. The greenhouse experiment revealed that clubroot severity in subsequent OSR was reduced after cropping the resistant cultivar. This study showed significant yield damage of P. brassicae already at low infestation levels.