To identify risk factors in the development of arm edema (AE) after conservative management of breast cancer, the authors prospectively measured differences in upper and lower arm circumference in 282 patients with stage I or II breast cancer who received radiation. AE was defined as a difference of 2.5 cm or more in either measurement between treated and untreated arms. Median follow-up was 37 months (range, 7-109 months). The crude frequency of AE overall was 19.5% (55 patients). In 21 patients (7.4%) AE was transient; 34 patients (12.1%) had persistent AE, which is the focus of this article. The 5-year actuarial incidence of persistent AE was 16%. The crude risk of persistent severe AE was 3.9%. Various factors were examined for their ability to enable prediction of AE. Treatment-related factors did not significantly enable prediction of AE, whereas factors related to patient size, such as body mass index, were strongly associated with both the frequency and severity of AE.