A patient treated with cyclophosphamide for breast cancer developed functional and clinicoradiological signs of sub-acute diffuse interstitial pneumopathy. Bronchoalveolar lavage revealed lymphocyte alveolitis. Differential diagnoses were excluded and the course was favorable after cyclophosphamide withdrawal. The bronchoalveolar lavage results obtained initially and at follow-up and two previous lavages reported in the literature demonstrate the importance of this examination in the diagnosis of drug-induced pneumopathy.