In 1994, the treatment of ductal carcinoma in situ of the breast remains a controversial subject. There are two reasons for this: first, there are many possible treatments and secondly, there is still much discussion on choice criteria. Possible options are: Total mastectomy which or without axillary dissection. Tumorectomy followed by locoregional radiation therapy. Simple tumorectomy. Tamoxifen can be added to each of these options. The choice criteria are based on our knowledge of the complex natural history of these cancers. In this area some of the current concepts must be seriously revised. All of the former concepts were based on lesions observed at the time of their elaboration. These were voluminous tumours, diagnosed late with no prior conservative treatment. However, today, the lesions observed are quite different. We see "young" lesions which are naturally much smaller. It would appear that the natural history of small sized in situ ductal carcinoma of the breast is quite different from that of large sized tumours. The lesions seen today are less multifocal and less multicentric in nature than is generally thought. The progression of a small tumour is essentially segmentary. Likewise, the risk of occult micro-invasion is certainly less than previously thought. Even if the risk of micro-invasion still does exist, the risk of axillary lymph node invasion (resulting from an unknown infiltrating zone of tumoural tissue) is much lower. Finally the dogma of radioresistance, which was not based on any carefully conducted radiobiological study, has lost its substance since recent results on radiosurgery combinations have been published.(ABSTRACT TRUNCATED AT 250 WORDS)