Eighty cases of duct carcinoma in situ (DCIS) of the breast have been investigated by a cohort-retrospective study. These consisted of 8.5 per 1,000 of 9,446 breast biopsies originally diagnosed as benign, between 1964 and 1976, with a mean follow-up of 17.5 years. There were forty-one cases (51%) of DCIS of clinging type (CC); 30 cases (37.%) of CC associated with other types of DCIS; nine cases of DCIS other than CC two of which were DCIS of comedo-type. Invasive duct carcinoma (IDC) subsequently developed in 11 patients (14%), whereas DCIS recurred in 5 (6%). The recurrence was ipsilateral in 12 of these 16 patients. IDC appeared more frequently, with high statistical significance, when the lesion present in the original biopsy showed pleomorphic (P) nuclei (ie, poorly differentiated cyto-nuclear morphology). The Standardized Morbidity Ratio (SMR) was 8.0 (95% CI; 2.9-17.5) with the general population as reference. IDC that developed following a lesion displaying P nuclei also showed a statistically significantly more aggressive behavior. It is suggested that when cases of DCIS are followed-up for a considerable length of time, a two-wave pattern of aggressiveness becomes apparent. IDC that develops after a poorly differentiated DCIS leads to death more precociously than that appearing after other types of DCIS, especially those showing more bland nuclear cytology.