From January 1, 1976 to December 30, 1985, 1,966 cases of breast carcinoma were diagnosed and treated at Malmo General Hospital, Malmo, Sweden. Of these cases, 185 (9.4%) involved invasive lobular carcinoma (ILC). Mammography in 137 cases demonstrated the following findings: spiculated opacity (53%), architectural distortion (16%), poorly defined opacity (7%), normal or benign findings (16%), and parenchymal asymmetry (4%). Radiographic definition of the ILC lesion varied greatly with projection: The craniocaudal view demonstrated significant findings more frequently than either the oblique or lateral views. Secondary radiographic findings were present in 31%, microcalcifications were rare, and physical findings were present in 89%. Because of its diffuse growth pattern and tendency to form lesions with opacity equal to or less than that of the parenchyma, ILC can be extremely difficult to detect mammographically. Therefore, the radiologist must be alert for subtle mammographic signs of malignancy and highly suspicious of any abnormal physical findings regardless of the mammographic appearance.