Idiopathic pulmonary fibrosis (IPF) is a rapidly progressive interstitial lung disease of unknown aetiology. Interleukin (IL)-1β plays an important role in inflammation and has been associated with fibrotic remodelling. We investigated the balance between IL-1β and IL-1 receptor antagonist (IL-1Ra) in bronchoalveolar lavage fluid (BALF) and serum as well as the influence of genetic variability in the IL1B and IL1RN gene on disease susceptibility and cytokine levels. In 77 IPF patients and 349 healthy controls, single nucleotide polymorphisms (SNPs) in the IL1RN and IL1B genes were determined. Serum and BALF IL-1Ra and IL-1β levels were measured using a multiplex suspension bead array system and were correlated with genotypes. Both in serum and BALF a significantly decreased IL-1Ra/IL-1β ratio was found in IPF patients compared to healthy controls. In the IL1RN gene, one SNP was associated with both the susceptibility to IPF and reduced IL-1Ra/IL-1β ratios in BALF. Our results show that genetic variability in the IL1RN gene may play a role in the pathogenesis of IPF and that this role may be more important than thought until recently. The imbalance between IL-1Ra and IL-1β might contribute to a proinflammatory and pro-fibrotic environment in their lungs.