Adenoid cystic carcinoma is a rare type of invasive breast carcinoma that has a good prognosis. We studied a series of four cases of adenoid cystic carcinoma in which we correlated the clinical and pathological features. The pathological features examined included light microscopy; electron microscopy; immunohistochemistry using antibodies to keratin, vimentin, S100 protein, actin, estrogen and progesterone receptors, and proliferation marker MiB-1, and p53 suppressor protein; image cytometric analysis for measurement of DNA ploidy; and molecular analysis using polymerase chain reaction single strand conformation polymorphism to assess point mutation of the p53 gene. All of the cases had a low nuclear grade, were negative for estrogen and progesterone receptors, and were DNA diploid. Three of the cases showed no evidence of metastases and had small primary tumors with low proliferative activity and absence of p53 protein expression. In contrast, one of the cases showed axillary lymph node metastases and in this case the primary tumor was large with a higher proliferative activity and expression of p53 protein, suggesting that these factors might play a role in the biological behavior of adenoid cystic carcinoma.