High rates of structural chromosome aberrations were associated with increased yields of sister chromatid exchanges (SCE) in metaphase chromosomes of a new born female calf affected by a congenital malformation. The frequency of abnormal cells was 25 per cent in the abnormal calf, 8 per cent in its dam and 3 per cent in a group of four healthy cows. Chromatid and chromosome breaks were the most frequent types of chromosome aberration found in the malformed calf; centric fusions, chromosome fragments and deletions were much less common. The mean rate of SCE/cell in the malformed calf was nearly twice that of the control and the difference was statistically significant. Possible factors involved in the occurrence of such a malformation are discussed.