Radionuclide bone scans performed in some patients with carcinoma of the breast showed abnormal retention of isotope in the renal pelvis which developed and progressed during the period of follow-up after mastectomy. This phenomenon was seen in 15 of 30 patients with infiltrating carcinoma of the breast (ILC) but in none of 29 with infiltrating duct carcinoma (IDC). Autopsies were performed in eight of the patients with ILC and three of those with IDC. Diffuse retroperitoneal and ureteric infiltration was seen in seven (88 per cent), with ILC and none with IDC. These data suggest that this scan abnormality is an indicator of retroperitoneal spread by ILC.