Screening mammography is underutilized, even for women older than 50 years for whom there is a general consensus that regular annual screening is appropriate and necessary. To evaluate reasons for this underutilization, we studied a random sample of 517 women in Los Angeles, Calif who were older than 50 years. All women were found to be underscreened, especially women older than 65 years. For example, approximately 35% of women 50 to 64 years old and 47% of women aged 65 years and older never had had even one mammogram. Analyses revealed that the most important factor that predicted whether a women ever had had a mammogram was whether her physician had talked to her about mammography. Women were between four and 12 times more likely, depending on their age group, to have had a mammogram at some time if their physicians discussed it with them. The discussions did not need to be lengthy or complex. These results indicate that physicians need to know that discussing screening mammography with their patients has a major impact on breast cancer screening behaviors.