Interleukin-4 (IL-4), a pleiotropic cytokine, has in vitro activity against non-Hodgkin lymphoma (NHL). This phase II study was conducted to learn the efficacy and toxicity of IL-4 in patients with NHL. Patients with relapsed or refractory indolent or aggressive NHL were eligible to receive 2.5 or 5.0 mcg/kg of subcutaneous IL-4 for 28 days of a 42-day cycle. Patients with response and acceptable toxicity after two cycles were eligible to continue treatment for six cycles. The target overall response rate (ORR) was 20%. Forty-one patients were enrolled and assessable for toxicity; two were ineligible after histology review. The ORR was 13% (5/39) with one complete and four partial responses. All responders were treated with 5.0 mcg/kg; the median time to progression was 84 days, the median duration of response for responders was 8.3 months. The most common toxicities of any grade in all patients were edema (66%), malaise (56%), and elevated liver function tests (56%). Grade 3 and 4 toxicities were more common at 5.0 mcg/kg, leading to a reduction in the starting dose. Although the study observed anti-tumor activity with IL-4, the ORR goal of the study was not achieved. Agents that target the IL-4 receptor can potentially benefit patients with NHL; however, alternative schedules using IL-4 in shorter duration and in combination with other agents would be required to overcome toxicities observed in this study.