Idiopathic pulmonary fibrosis (IPF) is a devastating lung disease with a median survival of three years. IPF deteriorates upon viral or bacterial lung infection although pulmonary infection (pneumonia) in healthy lungs rarely induces fibrosis. Bacterial lipopolysaccharide (LPS) activates Toll-like receptor 4 (TLR4) initiating pro-inflammatory pathways. As TLR4 has already been linked to hepatic fibrosis and scleroderma, we now investigated the role of TLR4 in IPF fibroblasts. Lung tissue sections from IPF patients were analyzed for TLR4 expression. Isolated normal human lung fibroblasts (NL-FB) and IPF fibroblasts (IPF-FB) were exposed to LPS and TGF-β prior to expression analysis of receptors, pro-fibrotic mediators and cytokines. TLR4 is expressed in fibroblast foci of IPF lungs as well as in primary normal human lung fibroblasts (NHLF), NL-FB, and IPF-FB. As a model for a gram-negative pneumonia in the non-fibrotic lung, NL-FB and IPF-FB were co-exposed to LPS and TGF-β. While NL-FB produced significantly less connective tissue growth factor (CTGF) upon co-stimulation compared to TGF-β stimulation alone, IPF-FB showed significantly increased pro-fibrotic markers compared to control fibroblasts after co-stimulation. Although levels of anti-fibrotic prostaglandin E2 (PGE2) were elevated after co-stimulation, they were not responsible for this effect. However, significant downregulation of TGF-β receptor type 1 in control fibroblasts seems to contribute to the reduced pro-fibrotic response in our in vitro model. Normal and IPF fibroblasts thus differ in their pro-fibrotic response upon LPS induced TLR4 stimulation.