The authors reviewed their experience with 542 patients with breast cancer who were treated with conservative surgery and radiation therapy (CSRT) and analyzed the outcome in those patients whose tumors could not be detected with mammography. Fifty-five of the patients (10.1%) had a palpable, pathologically confirmed breast carcinoma and a negative preoperative mammogram. Routine follow-up included annual mammography and physical examination. The local recurrence, 5-year actuarial survival, and 5-year disease-free survival rates for these 55 patients did not differ significantly from those for patients with positive mammograms. There were six cases of local breast recurrence in this subgroup. Four of five cases were visible on mammograms (one patient did not undergo mammography at the time of recurrence); two of the cases were detected with mammography alone following physical examination with negative results. The authors conclude that patients with palpable but mammographically occult early-stage breast cancer are suitable candidates for CSRT and that mammography is a mandatory part of follow-up of conservatively treated patients.