PURPOSE: Histopathological classification of invasive breast carcinoma with its earliest phases is fraught with pitfalls. We were willing to clarify the biology and clinicopathological features of microinvasive carcinoma which is not fully understood in comparison with those of in situ cancer. Particular attention is paid to identifying the novel markers which can be representative of the 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 out. Their paraffin blocks were chosen for immunohistochemical staining against four molecules. RESULTS: Microinvasive carcinoma was greater in primary tumor size (2.66?0.17cm vs 2.21?0.19cm, P=0.045) and metastatic axillary nodes (0.21?0.25 vs 0.06?0.16, P=0.019) than DCIS. In terms of nuclear grade(P=0.198) and comedo type(P=0.562), there was no statistical significance between microinvasive carcinoma and DCIS. Among three primary tumor features(size, comedo component, and nuclear grade), the tumor size> or =2.5cm had marginal significance affecting the incidence of axillary node metastasis in microinvasive carcinoma(P=0.081). Of investigational prognostic factors, determined by immunohistochemical staining, p53 expression was observed more frequently in microinvasive disease entity from in situ to invasive from than DCIS(P=0.031). CONCLUSION: Microinvasive carcinoma is thought to be transitional disease entity from in situ to invasive form. The microinvasive carcinoma of 2.5cm could be indication for axillary node dissection. In addition, p53 mutation might play a important biological role in progression from noninvasive to invasive form and these results provide further evidence that p53 mutation could have potential use as a molecular marker.