Background. Decisions to refer patients to other physicians for care or consultation are an important component of the provision of appropriate care for cancer patients. However, little is known about the referral process between specialists. Objectives. To examine the referral patterns of specialists to specialists and to understand why only some breast cancer patients receive a consultation with a medical oncologist. Research Design. This study was conducted in a large metropolitan region from 1993 to 1995 using a 2-staged population-based sampling strategy. One hundred seven physicians discussed 244 patient cases and their own knowledge, attitudes, and practices toward treatment and referral. Results. Of the 244 patients, 87.7% were referred to an oncologist, and 10.2% were actually prescribed tamoxifen by their surgeons before they saw the oncologist. Surgeons who were less involved in making decisions about the type of adjuvant therapy the patients were to receive and who preferred the use of chemotherapy were significantly more likely to refer patients to oncologists. Patients who were older, unemployed, node negative, and had a better prognosis or preferred not to see an oncologist were significantly less likely to be referred. These 7 factors explained a total of 55% of the variation in surgeons' decisions to refer patients to an oncologist. Conclusions. Extramedical factors, such as surgeon and patient preferences and communication factors, play a strong role in the referral process. In this sample, most patients were referred to an oncologist. However, older, unemployed patients and patients whose medical features indicated a better long-term prognosis were most likely to be among the nonreferred group.