Local wound environment under oxygen-permeable and oxygen-nonpermeable dressings in patients with chronic ulcers was investigated. The oxygen tensions under both these dressings were very low or zero. Wound fluid was more acidic under the nonpermeable hydrocolloid dressing than under the oxygen-permeable polyurethane dressing. Bacterial growth studied in vitro was retarded at the more acidic pH similar to that found under the hydrocolloid dressing. Viable and functioning neutrophils were found under both the polyurethane and hydrocolloid dressings, with a greater percentage of viable cells under the polyurethane film. Our data suggest that these synthetic dressings create hypoxic conditions in which wound healing occurs whether or not the dressing is permeable to oxygen. Furthermore the local wound environment can be modified by use of synthetic dressings.