Three cases of invasive mammary carcinoma with stromal osteoclast-like giant cells are reported. All the specimens were grossly of reddish-brown colour. On microscopical examination two were invasive ductal carcinomas and the third was of mixed ductal and mucoid type. In the stroma all specimens showed numerous osteoclast-like giant cells, vascular proliferation, and accumulation of haemosiderin pigment. Supplementary immunohistochemical and electron microscopical investigations indicated that the giant cells originated from mononuclear stromal cells with only slight histiocytic properties. The giant cell formation was probably induced by the infiltrating carcinomatous tissue which is also believed to induce new blood vessel formation. However, there is no evidence of a direct relationship between these features.