PURPOSE: The American College of Surgeons Oncology Group trial Z0011 was a prospective, randomized, multicenter trial comparing overall survival between patients with positive sentinel lymph nodes (SLNs) who did and did not undergo axillary lymph node dissection (ALND). The current study compares complications associated with SLN dissection (SLND) plus ALND, versus SLND alone. PATIENTS AND METHODS: From May 1999 to December 2004, 891 patients were randomly assigned to SLND + ALND (n = 445) or SLND alone (n = 446). Information on wound infection, axillary seroma, paresthesia, brachial plexus injury (BPI), and lymphedema was available for 821 patients. RESULTS: Adverse surgical effects were reported in 70% (278 of 399) of patients after SLND + ALND and 25% (103 of 411) after SLND alone (P <or= .001). Patients in the SLND + ALND group had more wound infections (P <or= .0016), seromas (P <or= .0001), and paresthesias (P <or= .0001) than those in the SLND-alone group. At 1 year, lymphedema was reported subjectively by 13% (37 of 288) of patients after SLND + ALND and 2% (six of 268) after SLND alone (P <or= .0001). The difference between the two groups' lymphedema, assessed by arm measurements at 30 days (P = .36), 6 months (P = .22), and 1 year (P = .078), although close to the cutoff for significance at 1 year, was not significant. BPIs occurred in less than 1% of patients. CONCLUSION: In trial Z0011, the use of SLND + ALND resulted in more wound infections, axillary seromas, and paresthesias than SLND alone. Lymphedema was more common after SLND + ALND but was significantly different only by subjective report. The use of SLND alone resulted in fewer complications.