At least two types of small metacentrics, which are isochromosomes for 12p and either 4p or 5p, respectively, are significantly associated with certain types of cancer and their formation may represent important stages in the development of these tumours. The specificity of the i(12p) for testicular cancer is now well established (it may also be present, however, in dysgerminomas and mixed Müllerian tumours of the ovary). This review is therefore mainly concerned with another marker, probably an i(5p) although an i(4p) should also be considered. Recent data suggest that this marker represents a significant chromosomal change occurring with a fairly high frequency in a variety of cancers, including carcinomas of the cervix, ovary, breast, bladder and bronchus (excluding small-cell carcinomas). These isochromosomes may contribute to tumour development through gene amplification; consistent with this is the frequent presence of these markers in two or more copies.