Four cases of alveolar variant of invasive lobular carcinoma of the breast were examined by electron microscopy, and their appearance compared with those of in situ and classical invasive lobular carcinoma. The individual tumor cells in the alveolar variant were basically similar to those described in the other two lobular tumors. The neoplastic alveoli consisted mostly of light cells, although a few dark cells were sometimes seen at the periphery. The nuclei were large, rounded and rich in euchromatin. The cytoplasm contained a variable amount of mitochondria and endoplasmic reticulum, and a few cells had intracytoplasmic filaments. Numerous tumor cells with membrane-bound electron-dense granules were occasionally seen. A few cells also had intracytoplasmic lumina. Tumor cells were usually seen in close contact with each other. Interdigitating processes and desmosomes were present. In general the tumor cells were not surrounded by basal lamina, but most alveoli were surrounded by elongated fibroblasts or myofibroblasts. These findings confirm both the lobular and the invasive nature of this distinct variant of breast carcinoma.