Twenty-three cases of ductal carcinoma in situ (DCIS), ten of which had an associated invasive component, were studied for loss of heterozygosity (LOH) of microsatellite markers on chromosome 9p and the results compared with a panel of 20 invasive breast carcinomas. In addition to the gene encoding p16, chromosome 9p is also thought to contain other putative tumour-suppressor genes. If the three panels of breast tumours showed LOH of markers in this region this would suggest that such putative genes were important in breast carcinogenesis. By studying both preinvasive and invasive breast tumours, it should also be possible to gain further information about the relationship between lesions of a different stage and to determine whether DCIS is indeed a precursor of invasive ductal carcinoma. Levels of LOH were low in the invasive-only set of tumours. Surprisingly, considerably higher levels of loss were observed in the tumours with an in situ component. Also, much heterogeneity was observed between different DCIS ducts or invasive tumour and DCIS from the same case.