BACKGROUND The natural history of patients with intraductal carcinoma (DCIS) and microinvasion is poorly defined, and the clinical management of these patients, with particular reference to management of the axilla, has been controversial. Previous studies of this lesion have used varied and/or arbitrary criteria for the evaluation of microinvasion. METHODS Thirty-eight DCIS lesions with microinvasion (n=29) or probable microinvasion (n=9), diagnosed during the period 1980-1996, were retrospectively analyzed after cases not treated with mastectomy and axillary lymph node dissection were excluded. Microinvasion was defined as a single focus of invasive carcinoma < or = 2 mm or up to 3 foci of invasion, each < or =1 mm in greatest dimension. RESULTS The patients were all females with a mean age of 56.4 years. DCIS was of comedo (n=31) or papillary (n=7) subtype. Microinvasion was often associated with an altered, desmoplastic stroma (55%) or a lymphocytic infiltrate (39%). The foci of microinvasion ranged from 0.25 to 1.75 mm (mean, 0.6 mm), with an aggregate mean size of 1.1 mm (range, 0.25-2.25 mm). Foci of microinvasion, ranging from 1 to 3 (mean, 1.7), were adjacent to DCIS in 95.3% of cases. The extent of DCIS did not correlate with the number of foci of microinvasion. Axillary lymph node dissections yielded a mean of 19.3 lymph nodes (range, 7-38), and all lymph nodes were negative for metastasis. None of 33 patients, followed for a mean of 7.5 years (range, 1.0-14.4 years), developed local recurrence or metastasis. CONCLUSIONS The cases of microinvasive carcinoma examined in this study, as defined above, were not associated with axillary lymph node metastases and appeared to be associated with an excellent prognosis. Further study is indicated to determine the appropriate management and long term prognosis of patients with this lesion.