To investigate the effect of L-arginine (L-arg) on the proliferation of human mesangial cells and production of collagen. The influence of L-arg on the cell proliferation was determined by MTT assay, immunocytochemical detection of expression of proliferative cell nuclear antigen (PCNA), and flow cytometrical analysis of cell cycle. Procollagen III and total collagen level in the supernatant and expression of collagen IV mRNA in human mesangial cells were determined by radioimmunoassay, hydroxyproline colorimetric assay, and reverse transcription polymerase chain reaction (RT-PCR). L-Arg induced inhibition of human mesangial cell lines (HMCL) in a concentration- and time-dependent manner. Immunocytochemical study for PCNA showed the number of cells was decreased, though the percentage of PCNA positive cells was increased in L-arg-treated group. Flow cytometrical analysis showed that cells in S and G2/-M phases were markedly increased in L-arg-treated group compared with those in control group. Furthermore, L-arg significantly inhibited the production of procollagen III and total collagen in the supernatants determined by radioimmunoassay and hydroxyproline colorimetric assay (P < 0.05 and 0.01, respectively) and inhibited the expression of collagen IV mRNA determined by RT-PCR (P < 0.01). L-arg could exert an inhibitory effect on the proliferation of human mesangial cells and production of extracellular components, which strongly suggested its potential therapeutic role in the chronic renal scarring.