Experimental evidence indicates that the lipid peroxidation of biological membranes is often associated with the development of liver fibrosis. We have studied the effect of neutrophil‐derived reactive oxygen species (ROS) on collagen synthesis by human hepatic stellate cells (HSC), the major source of collagen in the liver, in a coculture system. Lipid peroxidation in the cocultures was evaluated in terms of either malondialdehyde (MDA) production or the formation of MDA/4‐hydroxynonenal protein adducts. The expression of cellular messenger RNAs (mRNAs) was evaluated by either Northern blotting or RNAse protection assay. Nitric oxide (NO) synthase activity in cells was measured by [³H]citrulline formation from [³H]arginine. In vitro exposure of HSC to ROS resulted in the early induction of lipid peroxidation and was associated with a marked increase (threefold) of procollagen I mRNA expression and synthesis. The addition of antioxidants, such as vitamin E or superoxide dismutase (SOD), impaired this stimulation. The inhibition of neutrophil NO formation by N^G‐monomethyl‐L‐arginine made the ROS‐induced stimulation of procollagen I more evident. The addition of xanthine/xanthine oxidase X/XO, a superoxide anion donor, to HSC cultures strongly increased procollagen I synthesis. This stimulation was hampered by the addition of both SOD and sodium nitroprusside (an NO donor). The contribution of HSC to the production of NO in our coculture system was negligible, because inducible NO synthase (iNOS) mRNA was almost undetectable in these cells, and also because the amount of NO produced by HSC stimulated with tumor necrosis factor α (TNF‐α) and lipopolysaccharide (LPS) was 500 times less than that synthesized by neutrophils. In conclusion, these results indicate that neutrophil‐derived ROS may contribute to the development of hepatic fibrosis associated with alcoholic hepatitis. NO produced by neutrophils may exert a “protective” antioxidant effect by operating as a scavenger of superoxide anion.