The effect of nematode infections on growth performance during winter housing and on first lactation yield was investigated in heifers on 69 farms. Half of each herd was treated with albendazole after housing, which resulted in significantly reduced egg output, antibody titres against Ostertagia and Cooperia, and pepsinogen values compared with the untreated control groups. Albendazole-treated groups gained on average 0.007 cm day-1 heart girth (i.e. 0.05 kg day-1) more than control groups during the housing period (P less than or equal to 0.10), varying from -0.060 to +0.137 cm day-1 among herds. Albendazole-treated heifers produced 195.4 kg milk per 305-day lactation more than untreated heifers (P less than 0.01). Per herd, the mean milk yield response to treatment was +313.8 kg milk (P less than 0.01), varying from -876 to +1950 kg. Milk production of untreated heifers increased by 10.5 kg milk kg-1 extra body weight, adjusted for age, measured after the second grazing season (P less than 0.05). This suggests that nematode infections occurring in the first 2 years of life negatively influence milk production by reducing weight gains and hence body weight at calving.