In the present study, we assessed the clinical effect of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) in the treatment of refractory, grade III-IV hemorrhagic cystitis (HC) in six patients who underwent bone marrow transplantation (BMT). These were four males and two females, aged 24-40 years (median age 30.5 years). All received allogeneic BMT from HLA-identical siblings after preparation with busulfan-cyclophosphamide. HC was evident 24. 5 days (range 15-33 days) after BMT. Median duration of HC before treatment was 5 days (range 4-9 days). Treatment consisted of intravesical instillation of rhGM-CSF (400 microg) for 3 consecutive days. A complete response was observed in three patients, the other three showed a partial response. Median time to achieve response was 36 h (range 0.2-72 h). Hematuria was controlled after the first (two patients), second (two patients) or third (two patients) dose of intravesical rhGM-CSF. Patients were discharged from the hospital 10. 5 days (range 3-41 days) after treatment. All patients have been followed for up to 10 months and none have required further treatment. No systemic or bladder side-effects have been observed. Although our results indicate that intravesical instillation of rhGM-CSF is effective in the treatment of HC, a phase II clinical trial, including a larger series of patients, is needed.