The root lesion nematode causes economic losses in wheat and barley internationally, through both reduced grain yield and grain quality. This study investigated the relationships between the pre-sowing density and grain yield, and post-harvest nematode densities. Four field experiments were conducted at the same site between 2010 and 2014. A range of pre-sowing densities were established in the first year by growing three cereal cultivars that ranged from resistant to susceptible. In the following year, plots were over sown with the five same cereal cultivars. A linear relationship was observed between the natural log of the pre-sowing density and grain yield across all seasons. The results showed grain yield losses varied between cultivars and seasons. The importance of season was significant, with this study conducted over several seasons and highlighted the variability in yield losses between seasons which will need further investigation. The greatest yield losses observed were 25-28%, when the maximum pre-sowing densities ranged between 150-250 . g soil. An analysis of the relationship between the pre-sowing and post-harvest nematode densities revealed that increased pre-sowing nematode densities resulted in decreased multiplication rates in all seasons and in all cultivars. Nematode multiplication rates also varied between seasons. These results explain why it is difficult to predict nematode levels based on cropping history and highlight the importance of growing resistant cultivars to maintain low levels of to minimize risk of yield losses.