Summary. Respiratory virus infections can cause serious morbidity and mortality after conventional allogeneic stem cell transplantation. However, the incidence and outcome of these infections after reduced intensity conditioning has not been reported. Between 1997 and 2001, 35 episodes of respiratory virus infections were noted in 25 of 83 transplant recipients conditioned with fludarabine, melphalan and Campath-1H, and 80% of them received early antiviral therapy. Parainfluenza virus (PIV) 3 was the commonest isolate (45·7%) followed by respiratory syncytial virus (37%). Patients with myeloma were more susceptible to these infections [odds ratio (OR) 4·1, P = 0·01] which were often recurrent in patients with severe acute or chronic graft-versus-host disease (GVHD) (OR 10·6, P = 0·03). Infection within the first 100 d (OR 5·0, P = 0·05) and PIV 3 (OR 9·2, P = 0·01) isolation were risk factors for developing lower respiratory infection. Although more than half of the episodes progressed to lower respiratory infection, the mortality was only 8%. This could have been due to early initiation of antiviral therapy, but the attenuation of pulmonary damage due to the reduced-intensity conditioning, low incidence of GVHD and, paradoxically, the low CD4+ T-cell subset in this setting might also have been contributory factors.