BACKGROUND: Numerous studies have demonstrated that sentinel lymph node (SLN) biopsy can accurately determine axillary nodal status for breast cancer, but unacceptably high false negative rates have also been reported. Attention has been focused on factors associated with improved accuracy. We have previously shown that injection of blue dye in combination with radioactive colloid reduces the false negative rate compared with injection of blue dye alone. We hypothesized that this may be from the increased ability to identify multiple sentinel nodes. The purpose of this analysis was to determine whether removal of multiple SLNs results in a lower false negative rate. STUDY DESIGN: The University of Louisville Breast Cancer Sentinel Lymph Node Study is a prospective multiinstitutional study. Patients with clinical stage T1-2, N0 breast cancer were eligible for enrollment. All patients underwent SLN biopsy using blue dye alone, radioactive colloid alone, or both agents in combination, followed by completion level I and II axillary dissection. RESULTS: A total of 1, 436 patients were enrolled in the study from August 1997 to February 2000. SLNs were identified in 1, 287 patients (90%), with an overall false negative rate of 8.3%. A single SLN was removed in 537 patients. Multiple SLNs were removed in 750 patients. The false negative rates were 14.3% and 4.3% for patients with a single sentinel node versus multiple sentinel nodes removed, respectively (p = 0.0004, chi-square). Logistic regression analysis revealed that use of blue dye injection alone was the only factor independently associated with identification of a single SLN (p < 0.0001), and patient age, tumor size, tumor location, surgeon's previous experience, and type of operation were not significant. CONCLUSIONS: The ability to identify multiple sentinel nodes, when they exist, improves the diagnostic accuracy of SLN biopsy. Injection of radioactive colloid in combination with blue dye improves the ability to identify multiple sentinel nodes compared with the use of blue dye alone.