Lobular carcinoma in situ (LCIS) was diagnosed in 165 surgical specimens (119 patients) at our institution between 1974 and 1987. LCIS was seen more often in younger women (mean age, 49 years) than other breast carcinomas were (mean age, 58 years). Sampling of a single breast revealed multifocal disease in 70% (96/138). When both breasts were sampled, bilateral foci were found in 50% (41/82). Of 165 breasts with foci of LCIS, 37% (61/165) had simultaneously occurring invasive cancers in the same breast. Direct mammographic-pathologic correlation of foci of LCIS was possible in 73 breasts (67 patients). Microcalcifications were an indication for biopsy in 49% (20/41) of breasts with a mammographic abnormality, but were a nonspecific finding often found in tissues adjacent to foci of LCIS. The mammogram was normal in 44% (32/73) of breasts with foci of LCIS. The mammograms of patients with LCIS and those from a group of age-matched control subjects were compared by using a modified form of Wolfe's criteria and the percentage of fibroglandular elements. LCIS was seldom found in an N1 breast (1% vs 29%) or in a breast with less than 25% of its parenchymal area occupied by fibroglandular density (3% vs 33%). Compared with the control group, breasts with LCIS had more than 50% fibroglandular density (85% vs 45%) and a much higher frequency of the DY pattern (56% vs 36%). More fibroglandular density was seen in the LCIS group at all ages. Postmenopausally, the frequencies of the DY pattern and fibroglandular density greater than 50% in LCIS patients were nearly double those in the control group. LCIS patients have disease of the entire breast parenchyma, characterized by multifocality and bilaterality of various forms of lobular disease. Their mammograms reveal a higher rate of the DY pattern and higher percentages of fibroglandular or parenchymal density than those of age-matched controls. In LCIS patients, persistence of the DY pattern, or large amounts of fibroglandular density postmenopausally supports the concept that mammographically dense breasts are a marker for increased cancer risk in women 50 years old and older.