BACKGROUND: The histopathological classification of an invasive breast carcinoma in its earliest phases is fraught with pitfalls. We wanted to clarify the biology and the clinicopathological features of a microinvasive carcinoma, which are not fully understood, by comparing then with those of an in-situ cancer. Particular attention was paid to identifying the novel markers which might be representative of a microinvasive carcinoma. METHODS: From January 1986 to December 1996, a total of 72 microinvasive carcinomas, defined as in-situ carcinomas with invasion present in less than 10% of the histological section, were found. Their paraffin blocks were chosen for immunohistochemical staining against four molecules. RESULTS: Microinvasive carcinomas had a greater primary-tumor size (2.66+/-0.17 cm vs 2.21+/-0.19 cm, p=0.045) and a larger number of metastatic axillary nodes (0.21+/-0.25 vs 0.06+/-0.16, p=0.019) than DCIS (Ductal carcinoma in situ). In terms of nuclear grade (p=0.198) and comedo type (p=0.562), there were no statistical significances between microinvasive carcinomas and DCIS. Among three primary- tumor features (size, comedo component, and nuclear grade), a tumor size> or =2.5 cm had a marginal significance affecting the incidence of axillary-node metastasis in microinvasive carcinomas (p=0.081). Of the investigational prognostic factors determined by using immunohistochemical staining, p53 expression was observed more frequently in microinvasive tumors than in DCIS (p=0.031). CONCLUSION: A microinvasive carcinoma is thought to be transitional disease entity between the in-situ to the invasive forms. In spite of the marginal statistical significance of the result a microinvasive carcinoma larger than 2.5 cm could be an indication for axillary-node dissection. In addition, p53 mutation might play an important biological role in the progression from a noninvasive to an invasive form. Also the results provide further evidence that p53 mutation might have potential use as a molecular marker.