We previously reported mast cell increases in H. pylori gastritis. To determine the mechanism, we investigated the kinetics of mast cells and mast cell growth factor (stem cell factor, SCF) in H. pylori-positive and -negative gastric mucosa. Biopsy specimens from 12 H. pylori-negative and 28 positive subjects were examined. Sections were stained for mast cells, proliferating cell nuclear antigen (PCNA), and SCF. Densities of mast cells, PCNA-positive mast cells, and SCF-positive cells were significantly greater in H. pylori-positive than -negative subjects. SCF was expressed in mast cells and fibroblasts. The density of SCF-positive fibroblasts increased in H. pylori-positive gastritis and decreased after cure of infection. SCF mRNA was detected in H. pylori-positive gastric mucosa. Fibroblasts isolated from the normal gastric mucosa expressed SCF mRNA after incubation with H. pylori water extract. SCF may be one of the factors for mast cell increase. Fibroblasts may participate in mast cell increase and inflammation in H. pylori infection.