Bronchiolitis obliterans syndrome (BOS) is the major complication limiting survival of lung transplant recipients (Tx patients). The mechanisms underlying this fibrotic process are not known. We assessed IGF-1 and IGFBP-3 expression, critical mediators in different models of pulmonary fibrosis, in nine Tx patients. Three of them developed a BOS at 8, 14, and 17 mo postgraft, respectively. Two of the remaining six displayed a recurrent cytomegalovirus (CMV) infection, and four are in stable condition. IGF-1 mRNA expression was quantitated by RT-PCR in cells from four to six BAL per patient performed during the first 6 mo postgraft. Contrasting with a constantly low expression of IGF-1 mRNA in BAL cells from the six patients without BOS, the three patients with BOS presented marked peaks of IGF-1 on two to five occasions during the study period. These peaks, 3- to 13-fold increased compared with values from the former patients, preceded the diagnosis of BOS by 7, 13, and 17 mo, respectively. On the other hand, IGFBP-3 was highly and exclusively expressed in the three patients with BOS, the mRNA as well as the gene product as demonstrated by Western blotting. Our data strongly argue for a role of IGF-1 and IGFBP-3 in the fibrotic process underlying BOS, and for their possible value as an early marker of this complication.