Tubulolobular carcinoma is a type of mammary carcinoma that displays an admixture of invasive tubules and lobular-like cells. Previous reports have shown it to share clinical similarities to lobular carcinoma, whereas more recent studies have shown it to be E-cadherin positive. The aim of the current study was to further explore the immunophenotype of tubulolobular carcinoma, and to document its natural behavior. Nineteen cases of tubulolobular carcinoma and 10 cases each of tubular and lobular carcinoma were retrieved for comparison analysis. Immunohistochemistry was performed with antibodies against estrogen receptor, progesterone receptor, HER2/neu, 34betaE12, E-cadherin, and the catenins. Twenty-five percent of patients with tubulolobular carcinoma presented with greater than stage I disease, compared to 0 and 60% of patients with tubular and lobular carcinoma, respectively. Two patients with tubulolobular carcinoma had tumor recurrence, one of whom also developed metastasis. The majority of all carcinomas were estrogen and progesterone receptor positive. E-cadherin displayed membranous staining in all tubular and tubulolobular carcinomas, and was negative in all lobular carcinomas. Half of each carcinoma subtype displayed granular cytoplasmic 34betaE12 immunoreactivity. alpha-Catenin exhibited partial or complete membranous staining in all tubulolobular and tubular carcinomas, and was negative in all lobular carcinomas. beta-Catenin displayed membranous staining in tubulolobular and tubular carcinomas, whereas all lobular carcinomas had coarse cytoplasmic immunoreactivity. p120 and gamma-catenin displayed membranous staining in 100% of tubulolobular and tubular carcinomas and cytoplasmic staining in 100% of lobular carcinomas. Tubulolobular carcinoma of the breast is thus a distinct type of mammary carcinoma that displays both tubular and lobular patterns histologically but displays the membranous E-cadherin/catenin complex characteristic of the ductal immunophenotype. Tubulolobular carcinoma appears to be more aggressive than tubular carcinoma, as 16% of patients had lymph node metastases, although all were alive at a mean follow-up of 40 months.