BACKGROUND: Sentinel node (SN) biopsy has become the standard of care in the treatment of breast cancer. The aim of this study is to determine the value of additional tracer injection to increase the technical success rate of the SN procedure and to identify factors that influence the ability to visualize hotspots. METHODS: From February 1997 to August 2007, 1,208 clinically node-negative breast cancer patients underwent lymphatic mapping for SN biopsy. The technique involved the injection of 370 MBq (10 mCi) Tc-99 m-nanocolloid peritumorally. In case of insufficient or absent visualization of hotspots 37 MBq (1 mCi) of additional tracer was given intracutaneously above the tumor. RESULTS: In 93 patients (7.7%) visualization of hotspots on initial lymphoscintigraphy was insufficient (41 patients) or absent (52 patients). The first 14 patients did not receive additional tracer injection. In five patients, additional tracer did not result in successful lymphoscintigraphy, which is correlated with massive nodal tumor infiltration. In 33 patients with negative initial lymphoscintigraphy, additional tracer injection resulted in secondary SN visualization. In 41 patients with faint hotspots on initial lymphoscintigraphy, additional tracer injection, by increasing nodal uptake, simplified accurate SN biopsy. Decreased radiotracer uptake in this group was associated with older age and high body mass index (BMI). CONCLUSIONS: Additional tracer injection following initial scan failure increases the success rate of lymphoscintigraphy during lymphatic mapping in breast cancer, without compromising accuracy. If additional tracer injection does not result in secondary SN visualization, gross nodal tumor involvement is often present and axillary lymph node dissection (ALND) is mandatory.