The bronchoalveolar lavage (BAL) is considered a basic technique as a diagnostic aid in Pneumology. However, one of the main problems faced by the clinician is the lack of standardization of the technique. This has been resolved through the drafting of international standards. The other problem is the lack of what might be called a "reference" BAL. In order to establish a reference BAL, we analyzed 203 BAL divided in two groups: a control group and a pathologic group, make up by extrinsic asthma, intrinsic asthma, pulmonary infections, diffuse interstitial pneumopathies, bronchopulmonary tumors and chronic bronchitis. We have studied both the cytologic and the biochemical component of the BAL. Among the biochemical markers, we have considered; carcinoembrionary antigen (CEA), tissular polypeptidic antigen (TPA), neuronal specific enolase (NSE), ferritin (FER), calcitonin (CT), ACTH, histamin (HIS) and prostaglandin (PGE2). In order to establish the reference values, we have used the modified Baye's theorema. The BAL that we obtained was the following: volume 20 ml, cells 35 x 10(5) cells/ml, macrophages 77%, lymphocytes 22%, neutrophils 4%, eosinophils 2%, CEA 14 ng/mg, TPA 84 U/g PT, NSE 5 ng/mg PT, FER 42 ng/mg PT, CT 15 pg/mg PT, ACTH 51 pg/mg PT, HIS 1.22 ng/mg PT, PGE2 35 pg/mg PT.