Ductal carcinoma in situ (DCIS) of the breast is commonly described as a premalignant lesion. Using PCR to amplify DNA from areas of tumour cells which have been microdissected from fixed material, we have studied the involvement of chromosome 1 in 19 cases of DCIS. A series of microsatellite repeat polymorphisms has been used to define regions of allelic imbalance and this has confirmed the involvement in DCIS of six of the regions previously implicated in studies of invasive breast tumours. This suggests that these regions may harbour tumour suppressor genes, the inactivation of which is important for the early stages of breast tumour development. Analysis of separate ducts from within the same tumour has revealed that the same genetic alterations are not necessarily present throughout the lesion. In addition we have found that in three cases where frank invasive carcinoma is also present, similar alterations can be detected in the in situ and invasive component.