The pathogenesis of idiopathic pulmonary fibrosis is multifactorial and characterized by progressive fibrosis and excessive accumulation of extracellular matrix in the interstitium of the lung, and driven by an imbalance between anti-fibrotic and pro-fibrotic factors leading to collagen deposition. In the present study we wanted to identify proteins involved in these processes, and performed high-resolution proteomic profiling of bronchoalveolar lavage (BAL) from IPF patients and controls. The proteomic analysis of BAL demonstrated that the complement system was highly differentially regulated in IPF patients as compared with controls.