Background: Sentinel lymph node biopsy (SLNB) has emerged as a reliable, accurate method of staging the axilla for early breast cancer. Although widely accepted for T1 lesions, its use in larger tumors remains controversial. This study was undertaken to define the role of SLNB for T2 breast cancer. Study Design: From a prospective breast sentinel lymph node database of 1,627 patients accrued between September 1996 and November 1999, we identified 223 patients with clinical T1-2N0 breast cancer who underwent 224 lymphatic mapping procedures and SLNB followed by a standard axillary lymph node dissection (ALND). Preoperative lymphatic mapping was performed by injection of unfiltered technetium 99 sulfur colloid and isosulfan blue dye. Data about patient and tumor characteristics and the status of the sentinel lymph nodes and the axillary nodes were analyzed. Statistics were performed using Fisher's exact test. Results: Two hundred four of 224 sentinel lymph node mapping procedures (91%) were successful. Median tumor size was 2.0 cm (range 0.2 to 4.8 cm). One hundred forty-five of the 204 patients had T1 lesions and 59 patients had T2 lesions. There were 92 pathologically positive axillae, 5 (5%) of which were not evident either by SLNB or by intraoperative clinical examination. The false-negative rate and accuracy were not significantly different between the two groups, but axillary node metastases were observed more frequently with T2 than with T1 tumors (p = 0.005); other factors, including patient age, prior surgical biopsy, upper-outer quadrant tumor location, and tumor lymphovascular invasion were not associated with a higher incidence of false-negative SLNB in either T1 or T2 tumors. Conclusions: SLNB is as accurate for T2 tumors as it is for T1 tumors. Because no tumor or patient characteristics predict a high false-negative rate, all patients with T1-2N0 breast cancer should be considered candidates for the procedure. Complete clinical examination of the axilla should be undertaken to avoid missing palpable axillary nodal metastases. (C) 2000 by the American College of Surgeons.