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In order to determine the diagnostic value of bronchoalveolar lavage (BAL) and transbronchial lung biopsy (TBLB) in cryptogenic organizing pneu- monia (COP) a prospective study was carried out. Thirty seven consecutive patients (20 males and 17 females) with clinicoradio- logical features of COP were enrolled in the study. The statistical analyses were completed in 35 cases. Twenty eight patients were diagnosed to have COP, all of them with a confirmatory biopsy. In seven cases, a different diagnosis was made. BAL cytological and phenotypical criteria considered for the diagnosis of COP were: a lymphocytosis of more than 25% (with a CD4/CD8 ratio less than 0.9); combined with at least two of the following data (foamy macrophages of >20%, and/or neutrophils of >5%, and/or eosinophils of >2% and <25%). TBLB speci- mens were classified as positive for COP if they showed: buds of granulation tis- sue within the centrilobular air spaces; infiltration of alveolar walls with chronic inflammatory cells; and preservation of alveolar architecture. BAL was performed in 34 patients; 17 cases were consistent with the final diag- nosis of COP (sensitivity 63%), and four cases were correctly classified as nega- tive (specificity 57%). BAL had a positive predictive value (PPV) of 85% and a negative predictive value (NPV) of 29%. TBLB was performed in 32 patients; it correctly identified COP in 16 cases (sensitivity 64%), and six cases were correctly classified as negative (specificity 86%). TBLB had a PPV of 94% and a NPV of 40%. The accuracy of the examinations, that is the probability of correctly diag- nosing both diseased and nondiseased patients by BAL or TBLB, was 62 and 69%, respectively. Our findings suggest that the combination of cytological bronchoalveolar lavage and histological transbronchial lung biopsy data obtained during a fibreoptic pro- cedure appears to be an effective method for the initial investigation in crypto- genic organizing patients pneumonia presenting with patchy radiographic shadows.