To independently evaluate the impact of axillary surgery type and regional lymph node radiation (RLNR) on breast cancer-related lymphedema (BCRL) rates in patients with breast cancer.From 2005 to 2018, 1,815 patients with invasive breast cancer were enrolled in a lymphedema screening trial. Patients were divided into the following 4 groups according to axillary surgery approach: sentinel lymph node biopsy (SLNB) alone, SLNB+RLNR, axillary lymph node dissection (ALND) alone, and ALND+RLNR. A perometer was used to objectively assess limb volume. All patients received baseline preoperative and follow-up measurements after treatment. Lymphedema was defined as a ≥ 10% relative increase in arm volume arising > 3 months postoperatively. The primary end point was the BCRL rate across the groups. Secondary end points were 5-year locoregional control and disease-free-survival.The cohort included 1,340 patients with SLNB alone, 121 with SLNB+RLNR, 91 with ALND alone, and 263 with ALND+RLNR. The overall median follow-up time after diagnosis was 52.7 months for the entire cohort. The 5-year cumulative incidence rates of BCRL were 30.1%, 24.9%, 10.7%, and 8.0% for ALND+RLNR, ALND alone, SLNB+RLNR, and SLNB alone, respectively. Multivariable Cox models adjusted for age, body mass index, surgery, and reconstruction type showed that the ALND-alone group had a significantly higher BCRL risk (hazard ratio [HR], 2.66; P = .02) compared with the SLNB+RLNR group. There was no significant difference in BCRL risk between the ALND+RLNR and ALND-alone groups (HR, 1.20; P = .49) and between the SLNB-alone and SLNB+RLNR groups (HR, 1.33; P = .44). The 5-year locoregional control rates were similar for the ALND+RLNR, ALND-alone, SLNB+RLNR, and SLNB-alone groups (2.8%, 3.8%, 0%, and 2.3%, respectively).Although RLNR adds to the risk of lymphedema, the main risk factor is the type of axillary surgery used.