Graft versus host reaction (GVH) is a major complication of allogenic marrow transplants. The GVH present is a pluri-visceral syndrome in which certain pulmonary disorders are recognised. Amongst these respiratory failure by bronchiolitis is not an exceptional presentation. The case reported here is of an 18 year old man who developed, immediately following a marrow graft for acute lymphoblastic leukaemia, a lethal obstructive respiratory failure after progression for 2 1/2 years. The respiratory function data (TVO with elevated residual volume (VR) and VR/Total lung capacity (CT) hypoxia which corrected on exercise with normocapnia then hypoxic hypercapnia; compliance normal at low frequency but fell at high frequency and inspiratory and expiratory resistance was raised: DLCO/VA was normal) allowed the confirmation of obstructive respiratory failure by disease of the small airways. The pathogenesis of CVH is equivocal. Recurring infections seem to play a role, favoured by iatrogenic factors such as chemotherapy and total body irradiation. In the case reported here the first pulmonary signs followed an episode of influenza with sero-conversion.