The clinical syndrome "Bronchiolitis Obliterans Organising Pneumonia" (BOOP) has to be considered in patients with a flu-like illness since some weeks, fine crackles, and on chest X-ray bilateral patchy infiltrates. There is no response to antibiotics. BOOP is essentially idiopathic, but associations to other conditions exist. Lung function is often restrictive; biochemistry is not pathognomonic. BAL shows a mixed cellular pattern. The gold standard for pathologic diagnosis is open or thoracoscopic lung biopsy. However, a BOOP pattern or reaction is often seen on histologic specimens without the clinical-radiologic features of the BOOP-entity. Therapy consists of corticosteroids, which have to be prescribed for a long time at a rather high dose. Recurrence is frequent, but prognosis is good. Evolution to respiratory insufficiency and death is rare and may occur in rapidly progressive BOOP. This study reports on 11 cases (6 males/5 females) of clinical-pathological BOOP-syndrome (mean age 58 yrs, range 17-73 yrs), with an unexpectedly high mortality rate of 36% (4 cases). The disease was idiopathic in 7, and was associated with intake of amiodarone (in 1), with past Mycoplasma pneumonia (in 1) and with connective tissue disease (in 2). There was a history of a flu-like syndrome, cough and dyspnea of a mean duration of 4 months (range 1 week to 8 months). Lung function was mostly restrictive or/and obstructive with a diffusing capacity ranging between 47 and 95% predicted; there was hypoxia in about half of the patients. Chest X-ray and computed tomography (CT) scan showed a patchy consolidation with linear opacities (unilateral in 4 patients, bilateral in 5) and/or a ground glass pattern (in 4 patients), and a focal pseudo-tumoral lesion (in 1). Bronchoalveolar lavage showed a variable pattern of mixed, or eosinophilic or neutrophilic alveolitis. Histologic diagnosis was based on open lung biopsy (in 3), on thoracoscopic biopsy (in 2), on transbronchial biopsy (in 2), on wedge resection of the nodular lesion (in 1) and on postmortem lung biopsy (in 3). One patient recovered spontaneously, 1 remained cured after resection of the focal lesion, 7 were treated with 16-125 mg methylprednisolone (of whom 3 had a temporary flare-up during tapering the corticosteroids and 2 died after 1 and 3 months due to infectious complications), 2 died due to rapidly progressive BOOP.