BACKGROUND: We hypothesize that mapping the lymphatic drainage of the arm with blue dye (axillary reverse mapping [ARM]) during axillary lymphadenectomy decreases the likelihood of disruption of lymphatics and subsequent lymphedema. METHODS: This institutional review board-approved study involved 360 patients undergoing sentinel lymph node biopsy (SLNB) and/or axillary lymph node dissection (ALND) from May 2006 to October 2011. Technetium sulfur colloid (4 mL) was injected subareolarly, and 5 mL of blue dye was injected subcutaneously in the volar surface ipsilateral upper extremity (ARM). Data were collected on variations in lymphatic drainage, successful identification and protection of arm lymphatics, crossover, and occurrence of lymphedema. RESULTS: A group of 360 patients underwent SLNB and/or ALND, 348 of whom underwent a SLNB. Of those, 237 (68.1%) had a SLNB only, and 111 (31.9%) went on to an ALND owing to a positive axilla. An additional 12 of 360 (3.3%) axilla had ALND owing to a clinically positive axilla/preoperative core needle biopsy. In 96% of patients with SLNB (334/348), breast SLNs were hot but not blue; crossover (SLN hot and blue) was seen in 14 of 348 patients (4%). Blue lymphatics were identified in 80 of 237 SLN incisions (33.7%) and in 93 of 123 ALND (75.4%). Average follow-up was 12 months (range, 3-48) and resulted in a SLNB lymphedema rate of 1.7% (4/237) and ALND of 2.4% (3/123). CONCLUSION: ARM identified substantial lymphatic variations draining the upper extremities and facilitated preservation. Metastases in ARM-identified lymph nodes were acceptably low, indicating that ARM is safe. ARM added to present-day ALND and SLNB may be useful to lesser rates of lymphedema.