The effect of sainfoin (Onobrychis viciifolia) hay consumption on the pathophysiology and local cellular responses of growing lambs during infection with Trichostrongylus colubriformis was investigated. Thirty-two lambs, 16 weeks of age, were allocated to 1 of 4 treatment groups (n=8) that were offered either grass (G) or sainfoin (S) hay while concurrently either infected (+), or not (−) with 12 000 L3 T. colubriformis larvae per week for 6 weeks. Liveweight gains were affected by diet (P=0·002) and reduced by infection (P<0·005). Faecal egg count was reduced in S+ compared to G+from days 35 to 42 (P=0·001); however, total egg output, worm burdens at day 42 and worm fecundity were similar between diets (P>0·05). Feeding sainfoin appeared to enhance immune cell development with tissue eosinophils, mast cells and pan T cells present in greater concentrations in S+ than in G+ animals. However, further studies are required to determine if the enhanced immune cell development is a consequence of a greater nutrient supply or a direct influence of sainfoin metabolites on local inflammatory responses to the gastrointestinal nematode T. colubriformis.