Background: Weight gain is associated with an increased risk of cancer and unfavorable outcomes after a cancer diagnosis. Women with an elevated body mass index (BMI) at diagnosis have worse quality of life after completing radiation therapy (RT) for breast cancer as well as increased recurrence rates and cancer-related death. In this retrospective study we investigated the impact of a patient’s BMI on RT acute toxicities and on recurrence rates in patients treated for early stage breast cancer. Methods: Charts of 397 consecutive patients with early stage breast cancer undergoing breast conservation treatment between 2008 and 2013 were retrospectively analyzed. Patients were divided into three groups, normal BMI, overweight (BMI 25-30 kg/m²) and obese (BMI > 30 kg/m²). Acute toxicities, including skin toxicity and fatigue, were scored using CTCAE version 4.0. Recurrence was assessed based on the patient’s last mammogram. The three patient groups were compared using ANOVA and a paired-test. Results: 42.3% of the patients were obese. 72.7% of the patients developed fatigue, 30.0% of the patients developed dry desquamation, and 24.7% of the patients developed moist desquamation. Obese women experienced significantly higher rates of dry skin desquamation (p = 0.000256) and moist skin desquamation (p = 5.86x10^-9) as compared to patients with a normal BMI. There was no significant association between fatigue and increasing BMI. 5.2% of patients in the study recurred. Compared with women of normal weight, women with increased BMI were more likely to experience a tumor recurrence (p = 0.0468). Conclusions: Increased BMI is associated with increased risk of acute skin toxicities during radiation treatment for early stage breast cancer. In addition, elevated BMI is associated with a higher rate of tumor recurrence. Strategies to promote weight loss before and during cancer treatment should be offered to patients to decrease treatment-related toxicity and rate of recurrence.