BACKGROUND: Sweet's syndrome is a rare extraintestinal manifestation of Crohn's disease that is usually treated by corticosteroids. Cyclophosphamide therapy has been shown to be effective in steroid-refractory Crohn's disease with extraintestinal manifestations. The mechanism of action remains obscure. Here, we report about a case of steroid-refractory Sweet's syndrome accompanying Crohn's colitis treated by cyclophosphamide. METHODS: At baseline and two weeks after initiating cyclophosphamide pulse therapy, clinical symptoms were evaluated and apoptosis in mononuclear cells of the colon mucosa was quantified via immunofluorescence TUNEL-labeling. Ongoing clinical follow-up lasts for more than three years. RESULTS: Cyclophosphamide pulse therapy resulted in complete resolution of luminal activity and extraintestinal manifestations. TUNEL-marked CD4(+), CD8(+) and CD68(+) cells in intestinal biopsies showed a 338% increase as compared to baseline. CONCLUSIONS: Cyclophosphamide therapy was highly effective in steroid-refractory Crohn's colitis accompanied by Sweet's syndrome for induction of remission. Furthermore, apoptosis of mononuclear cells in the colon mucosa, including CD68(+) macrophages as well as CD4(+) and CD8(+) cells, appears to be a component of the anti-inflammatory effect of cyclophosphamide in Crohn's disease.