We analyzed the autopsy records and autopsy histological slides of 261 patients with breast carcinoma who died at the Institute of Oncology, Ljubljana, from January 1972 to October 1989, with particular attention to the metastatic pattern of infiltrating lobular carcinoma (ILC) which we compared with infiltrating ductal carcinoma (IDC). In 226 of 261 patients who died with metastatic disease there were 25 cases of ILC, 195 cases of IDC, 4 cases of mixed IDC-ILC, and 2 cases of mucinous carcinoma. There was no statistically significant difference in frequency of metastases to common metastatic sites, such as the liver, bone, and pleura, with the exception of the lungs, in which IDC metastases prevailed (P less than 0.006). By contrast, a statistically highly significant prevalence of ILC metastases to the peritoneum/retroperitoneum, hollow viscera, internal genital organs, leptomeninges, and myocardium was found (P values of less than 0.006- less than 10(-6). The metastases to these sites were characterized by diffuse growth of neoplastic cells that infiltrated in a lymphoma or leukemia-like fashion. Such metastases may remain clinically silent for a long time, in spite of their extensiveness. The difference of metastatic pattern between ILC and IDC is insufficiently appreciated in most of the published studies on ILC.