Minute deletions of the B-group short arm have been demonstrated in five of six cases with clinical features suggestive of a deletion of chromosome 4 or 5, but with grossly normal chromosomes. A deletion as small as 8% of the short arm was detectable by each of five observers when randomly arranged cells from controls, cases with large deletions and the cases with suspected minute deletions were scored blindly for the presence of a deletion. Another two cases were shown to have deletions of only 4% of the short arm by measurement studies, although the deletion in these cases could not be detected visually in the blind study. The presence of the deletion was confirmed in one case by DNA-labelling studies showing that the putative deleted chromosome consistently had the replication pattern of chromosome 4. In another two cases measurement of the long arm confirmed that the deleted chromosome was a chromosome 5.