Summary Treatment for Crohn's disease is aimed at immunosuppression. Yet inherited disorders associated with defective innate immunity often lead to development of a Crohn's-like disease. We performed an open-label dose-escalation trial (4–8 μg/kg per day) to investigate the safety and possible benefit of granulocyte-macrophage colony-stimulating factor (GM-CSF) in the treatment of 15 patients with moderate to severe Crohn's disease. No patients had worsening of their disease. Adverse events were negligible and included minor injection site reactions and bone pain. Patients had a significant decrease in mean Crohn's disease activity index (CDAI) score during treatment (p<0·0001). After 8 weeks of treatment, mean CDAI had fallen by 190 points. Overall, 12 patients had a decrease in CDAI of more than 100 points, and eight achieved clinical remission. Retreartment was effective, and treatment was associated with increased quality-of-life measures. GM-CSF may offer an alternative to traditional immunosuppression in treatment of Crohn's disease.