Measurement of the B-group short arms from 25 cases and five balanced carriers with a Bp– chromosome showed a significant clustering of the size of the deleted short arm between 0.45 and 0.65 of the normal short-arm length. No differences in length were apparent among family members when the deletion was familial. The distribution of deletion size appeared similar for chromosome 4 and chromosome 5. No correlation was found between any clinical feature and the length of the deleted short arm. The results are discussed in relation to the origin of deletions – simple terminal or interstitial deletions or reciprocal translocations.