In 74 in situ breast cancers an immunohistochemical study for estrogen (ER) and progesterone (PR) receptors, proliferation index (PI), and c-erbB-2, p53, and bcl-2 overexpression was performed. Cases were categorized as ductal carcinoma in situ (DCIS) comedo: 24.3% of cases; DCIS non comedo: 27% of cases; DCIS cribriform: 5.4% of cases; lobular carcinoma in situ (LCIS): 16.3% of cases; mixed carcinoma in situ: 27% of cases. Quantitation of immunohistochemical results was obtained with an image analysis computerized system (CAS 200). The cutoff values used were: 10% of positive area for ER, PR, NEU, and bcl-2; 5% of positive area for p53; 13% of PI for proliferative activity. DCIS cribriform and LCIS displayed a higher positivity for ER (92.6 and 93.8% of cases); DCIS cribriform and DCIS non comedo a higher for PR (89 and 75.3%); DCIS comedo presented the highest values for PI (65.4%), NEU (72.8%), and p53 expression (37.3%). All DCIS cribriform and DCIS non comedo and 99.6% of LCIS expressed bcl-2. The results underscore the importance of biological characterization of breast carcinoma in situ with the aim to define lesions natural history.