Acquired lymphedema is a cancer sequela and a global health problem currently lacking pharmacologic therapy. We have previously demonstrated that ketoprofen, an anti-inflammatory agent with dual 5-lipoxygenase and cyclooxygenase inhibitory properties, effectively reverses histopathology in experimental lymphedema. We show that the therapeutic benefit of ketoprofen is specifically attributable to its inhibition of the 5-lipoxygenase metabolite leukotriene B4 (LTB4). LTB4 antagonism reversed edema, improved lymphatic function, and restored lymphatic architecture in the murine tail model of lymphedema. In vitro, LTB4 was functionally bimodal: Lower LTB4 concentrations promoted human lymphatic endothelial cell sprouting and growth, but higher concentrations inhibited lymphangiogenesis and induced apoptosis. During lymphedema progression, lymphatic fluid LTB4 concentrations rose from initial prolymphangiogenic concentrations into an antilymphangiogenic range. LTB4 biosynthesis was similarly elevated in lymphedema patients. Low concentrations of LTB4 stimulated, whereas high concentrations of LTB4 inhibited, vascular endothelial growth factor receptor 3 and Notch pathways in cultured human lymphatic endothelial cells. Lymphatic-specific Notch1−/− mice were refractory to the beneficial effects of LTB4 antagonism, suggesting that LTB4 suppression of Notch signaling is an important mechanism in disease maintenance. In summary, we found that LTB4 was harmful to lymphatic repair at the concentrations observed in established disease. Our findings suggest that LTB4 is a promising drug target for the treatment of acquired lymphedema.