Diabetic wounds are characterized by a prolonged wound healing process with insufficient formation of granulation tissue. Systemic hyperbaric oxygen therapy has been observed to improve the healing of these wounds. However, the mechanism(s) responsible for these findings are not yet fully elucidated. In the present study we have studied the in vitro effects of hyperbaric oxygen on proliferation of human fibroblasts from normal skin and from chronic foot ulcers in non-insulin-dependent diabetics. A 1-hour exposure to hyperbaric oxygen at oxygen pressures between 106 and 300 kPa (795 to 2250 mm Hg) increased the proliferation in both diabetic and normal fibroblasts. The stimulatory effect was dose-dependent, with a peak increase in cell proliferation at 250 kPa and 200 kPa for normal and diabetic cells, respectively. The effects were not due to hydrostatic pressure per se. These results suggest that hyperbaric oxygen could stimulate fibroblast activity in the diabetic wound, a finding that could explain the enhanced formation of granulation tissue seen clinically in wounds treated with hyperbaric oxygen. We also speculate that mechanisms other than just increased oxygen availability may be responsible for our findings.