Risk factors for cytomegalovirus viraemia and disease, the relation between viraemia and disease, effect of antiviral treatment, and T-helper cell response to cytomegalovirus antigen were analysed retrospectively among 279 patients who underwent bone marrow transplantation at Huddinge Hospital. Ninety-one of 279 (32.6%) patients developed viraemia. Donor and recipient pre-transplant serologic status and degree of acute graft-versus-host disease were independent risk factors for viraemia. Forty-nine patients (17.6%) developed cytomegalovirus disease and 44 of these patients had viraemia. Seventeen patients (6%) developed cytomegalovirus pneumonia and 14 of these patients had preceding viraemia. Among patients with viraemia, acute graft-versus-host disease and total body irradiation were risk factors for pneumonia. Antiviral treatment initiated within 7 d of development of viraemia was associated with lower risk for development of pneumonia (P < 0.05). Sixty-seven patients with viraemia were repeatedly tested by lymphocyte stimulation with cytomegalovirus antigen. No patient who developed cytomegalovirus pneumonia had measurable specific helper T-cell response at the time of viraemia detection compared to 42% of patients with other concurrent or subsequent cytomegalovirus disease, and 75% of patients without subsequent disease. We conclude that viraemia is a major risk factor for development of cytomegalovirus disease. Furthermore, early antiviral treatment based on detection of viraemia can be effective in preventing cytomegalovirus disease. The length of antiviral treatment might be decided through measurements of the helper T-cell response to cytomegalovirus antigen.