A sentinel node biopsy done at the time of initial tumor resection allows for a one-stage surgical procedure. In addition, sentinel node identification may be impaired when done after a previous tumor excision. This study evaluates the sentinel node biopsy in patients with nonpalpable breast cancer and assesses whether a sentinel node biopsy for mammographically suspect breast lesions is justified when preoperative needle biopsy is inconclusive for invasive malignancy. A sentinel node biopsy was done in 67 patients with nonpalpable breast lesions after injection of radioactive tracer (intraparenchymal in 35 and subdermal in 32) and blue dye (para-areolar). A preoperative core needle biopsy was positive for malignancy in 42 patients. Thirteen patients had positive cytology or ductal carcinoma in situ (DCIS). In 12 patients the needle biopsy was nondiagnostic, but the lesions remained highly suggestive of malignancy on mammography. Sentinel node biopsy was successful in 64 patients (96%). In these, the sentinel node was both radioactive and blue in 58 patients (91%). Only 4 of 13 patients with positive cytology or DCIS on preoperative needle biopsy and only 5 of 12 patients without a preoperative diagnosis had an invasive cancer after resection. Sentinel nodes were positive for nodal metastases in 9 of 49 patients (18%) with a successful sentinel node biopsy for invasive malignancy. None of the eight patients with DCIS had nodal metastases. The sentinel node procedure avoids the potential morbidity of an axillary dissection in more than 80% of patients with nonpalpable breast cancer. A sentinel node biopsy for mammographically detected suspect breast lesions is not justified without a preoperative histologic diagnosis of invasive breast cancer.