Platelet-derived growth factor (PDGF) was shown to modulate fibroblast activity in interstitial lung diseases like idiopathic pulmonary fibrosis (IPF). The role of PDGF in fibrosing mechanisms in histiocytosis X is unclear. Eight patients with histiocytosis X, five patients with idiopathic pulmonary fibrosis (IPF), and nine patients with no evidence of interstitial lung disease underwent bronchoalveolar lavage (BAL). The c-sis gene (a proto-oncogen encoding for the B-chain of PDGF) expression was measured by gene hybridization revealing an upregulated c-sis transcript in the group of histiocytosis X and patients, whereas no c-sis expression was detectable in the control group. The alveolar macrophage supernatants from histiocytosis X patients and from the control group were incubated with a human lung fibroblast cell line (WI-38). The mitosis rate was measured by tritiated thymidine incorporation and collagen production was estimated by determining the procollagen III peptide concentration in fibroblast supernatants. Tritiated thymidine uptake was increased 1.6 times in histiocytosis X compared with the control group (p < 0.01). Procollagen-III-peptide levels in fibroblast supernatants after incubation with alveolar macrophage supernatants from histiocytosis X were elevated 2.5 times compared with the control group (p < 0.01). Prior to incubation with the WI-38 cell line, the cell supernatant then was preincubated with nonpreserved anti-human PDGF (AA- and BB-chain) resulting in an 80% decrease of tritiated thymidine uptake and procollagen-III-peptide production in the group of histiocytosis X patients compared with native supernatants. No significant change in fibroblast activity was seen in the control group. Preincubation with nonpreservated Ki-T2 antibodies as pan T-lymphocyte marker did not show significant differences in both groups excluding unspecific antibody inhibition. These findings suggest increased PDGF production by alveolar macrophages in histiocytosis X patients. The PDGF is in part responsible for increased fibroblast replication and collagen production.