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The purpose of this study was to describe the clinical features at onset and outcome and the diagnostic approach in subjects with bronchiolitis obliterans-organizing pneumonia (BOOP). Over a 7-year period we observed 78 cases of biopsy-proven bronchiolitis obliterans-organizing pneumonia, in which well documented clinical and radiographic data were available. The final diagnosis of BOOP was validated when patients presented: (i) negative microbiological analysis on BAL fluid; (ii) a well documented improvement either spontaneous, or after steroid treatment or (iii) cases with progressive respiratory failure and increasing radiographic shadows, an open lung biopsy or autopsy that excluded other entities. There were 42 males and 36 females; the mean age was 61±12 years (range 12–85 years). Forty-two (54%) patients were current smokers, 25 (32%) had never smoked and 11 (14%) were ex-smokers. The clinical pattern at presentation of BOOP was more frequently similar to classical acute or sub-acute infectious pneumonia. Fever (63%), dyspnoea (58%) and dry cough (53%) were the typical symptoms on admission. A flu-like syndrome preceeding BOOP was observed in 21 cases (27%). Inspiratory crackles (78%) were the most typical finding at physical examination. However, 13% of the patients were asymptomatic and an abnormal chest X-ray film was the reason for seeking medical attention. Radiographically the most frequent pattern of BOOP was a unilateral consolidation (44%) with lower field predominance. A migratory behaviour was present in 22% of the cases. High-resolution computed tomographic (HR-CT) scan when performed, was more sensitive in detecting ground glass infiltrates, sub-pleural or peri-bronchovascular distribution or the presence of nodules or cavitation.