Clinicopathologic data on microinvasive carcinoma of the breast (MICB) as defined by the 1997 TNM criteria (T1mic < or = 1 mm) is scarce. Histologic slides of 109 cases from 1993 through 1997, in which microinvasion was either suspected or diagnosed initially, were reviewed. A double immunoenzyme-labeling technique using antismooth muscle actin and anticytokeratin antibody on the same section was used to confirm invasion in equivocal cases. All foci of invasion were measured by ocular micrometer. Twenty-one cases were confirmed to be MICB. The mean age of the patients was 60.9 years. Thirteen patients presented with mammographic abnormalities on routine examination (60.9%). MICB was ductal in 18 patients, including one tubular carcinoma, and was lobular in three patients. The mean number of invasive foci was two per patient (range, one to seven foci). The accompanying duct carcinoma in situ had high-grade nuclei and necrosis in 16 of 18 patients (89%), 13 of which (72%) were comedo-type. Two of the 15 patients had one positive axillary lymph node each (13.3%). Eleven patients underwent mastectomy, nine received radiation therapy, one received chemotherapy, and two underwent lumpectomy only. Median follow up was 28 months (range. 18-63 months). One patient had a chest wall recurrence of infiltrating duct carcinoma and another recurred with duct carcinoma in situ.