We4 have previously shown that dexamethasone (DX), interleukin (IL)-4, and IL-13 enhance the proliferation of proximal airway fibroblasts from patients with asthma. However, the proliferative and synthetic characteristics of distal lung fibroblasts in response to these mediators are unknown. In an ongoing study, six subjects with nocturnal worsening of asthma (mean [± SD] overnight fall in FEV1, 20.0 ± 2.7%) underwent bronchoscopy with transbronchial biopsy at 4:00 am (mean FEV1, 69 ± 4% predicted). Biopsy specimens were placed in Dulbecco modified Eagle medium supplemented with fetal bovine serum (10%), streptomycin (100 μg/mL), penicillin (10,000 U/mL), and gentamicin (100 μg/mL), and were cultured until > 50% confluency was established (ie, approximately 8 to 20 days). Cells were passaged up to four times for data collection. Immunostaining with vimentin (Dako; Carpenteria, CA), Ab-1 (Calbiochem; San Diego, CA), and α-smooth muscle actin (Dako) confirmed fibroblast identity. For determining fibroblast proliferation, freshly trypsinized cells were seeded in 96-well plates at 104 cells per well and were incubated with IL-4 (50 U/mL), IL-13 (10 ng/mL), and a combination of the two for 48 h in the presence and absence of DX (concentration range, 10−6 through 10−10 mol/L). Proliferation was determined via [3H]thymidine incorporation, and procollagen I production was determined using a sandwich enzyme-linked immunosorbent assay.