To investigate possible factors influencing ease of calving in heifers, records from 392 calvings of replacement heifers on 10 research farms were analysed. Sire breeds were classified as either British or Continental, the majority of the Continental sires used being Limousin. Calving difficulty was recorded on a 1-3 scale. The data were analysed by regression analysis, dropping factors until the minimum r.s.d. value was obtained. Heifers in-calf to Continental bulls were mated at heavier weights, had heavier post-calving weights and greater pregnancy gains. Continental sires increased birth weights (3 ± 0.6 kg), gestation length (3.8 ± 2.55 days) and calving difficulties (calving score 1.14 and 1.41 ± 0.063 for British and Continental sires respectively). Birth weight was the major factor influencing calving difficulty (0.04 ± 0.012) with pregnancy weight gain the next most important. The effect of calf sex, gestation length and sire breed on calving difficulty were non-significant. This can be explained by the influence of birth weight, which alone accounted for most of the variation in calving difficulty.