Six cases of lactating adenoma of the female breast diagnosed by fine needle aspiration (FNA) were reviewed. The FNA cytologic diagnostic features included a usually moderately cellular aspirate with an abundant foamy background material, intact epithelial lobules or acini and small groups and solitary epithelial cells that contained uniform nuclei, fine chromatin and prominent nucleoli. When present, the cytoplasm was finely vacuolated or wispy; many nuclei appeared stripped of their cytoplasm. These features were compared light microscopically with the cytopathologic features of six cases of invasive well-differentiated ductal adenocarcinoma, seven cases of invasive lobular carcinoma, one case of granulocytic sarcoma and one case of primary histiocytic lymphoma of the breast. In addition, cytomorphometric analysis demonstrated no statistically significant differences in the nuclear areas of lactating adenoma as compared with those of well-differentiated ductal carcinoma and lobular carcinoma.