We describe a 10 year old boy with organising pneumonia associated with acute Mycoplasma pneumoniae infection. The diagnosis of organising pneumonia was made by open lung biopsy and the M pneumoniae infection was proven serologically. Antibiotic and long term corticosteroid treatment resulted in steadily improving pulmonary function monitored by spirometry. The introduction of antiinflammatory treatment with NSAIDs/immunosuppressive agents in order to spare steroids was well tolerated and resulted in further improvement of the pulmonary function. To our knowledge this is the first documented case of Mycoplasma pneumoniae associated organising pneumonia to be reported in a child.O rganising pneumonia, also described as bronchiolitis obliterans organising pneumonia (BOOP) is a rare disorder, usually occurring in patients over 50 years of age (range 20-80 years), with an equal sex distribution. There are only a few cases reported in adolescents. This rare disorder can be divided into cryptogenic (idiopathic) organising pneumonia (COP) of unknown cause, and secondary organising pneumonia associated with many recognised conditions, for example, infections, drug reactions, malignancy, radiation therapy, or autoimmune diseases. The pathological pattern is not specific for any disorder or cause, but reflects one special type of inflammatory process resulting from lung injury.1 It is characterised by patchy inflammatory changes of the bronchoalveolar lumen and wall, with some associated peribronchial scarring. The characteristic histological feature is the presence of buds of granulation tissue in the distal airspaces and interstitial mononuclear cell infiltrate.Various infectious agents have been described as rare causes of organising pneumonia.2 To our knowledge the association of Mycoplasma pneumoniae infection and development of organising pneumonia in children has not been reported to date. CASE REPORTA 10 year old boy presented with a six week history of dyspnoea on even mild exertion, fatigue, weight loss, and dry cough. These symptoms showed steady worsening that prompted hospital admission. The patient had an uneventful medical history apart from known hay fever. There were no cardiovascular, autoimmune, or pulmonary diseases in the past or in the family history.Physical examination revealed tachypnoea, dyspnoea with reduction of the oxygen saturation below 80% after mild exercise, and fine diffuse crackles on lung auscultation. Spirometry revealed a severe restrictive ventilation defect with the forced vital capacity (FVC) impaired to 25% of reference values ( fig 1). Chest roentgenogram and high resolution computed tomography (HR-CT) scan images showed notable alveolar and interstitial opacities with bilateral infiltration of the lung parenchyma (fig 2). Laboratory studies showed normal full blood count, negative C reactive protein, and slightly raised erythrocyte sedimentation rate (20/40 mm). Serological assays revealed raised M pneumoniae antibody titres (IgM 18 U/ml, IgG 80 U/ml, ELISA) indicating a recen...