Maggot therapy (biosurgery) has received increasing interest for the debridement of chronic wounds and for the improvement of wound healing. The purpose of this study was to investigate the clinical effects, side-effects, and possible mechanisms of action of biosurgery. Biosurgery was used for debridement in 30 patients with chronic leg ulcers of mixed origin. The effect of a single application of maggots for 1-4 days was evaluated by a clinical wound score and contact-free spectroscopy. Side-effects were recorded. Debridement was rapid and selective. The wound secretion was temporarily increased. We observed a significant improvement of the wound score with a decrease from 13.5 +/- 1.8 to 6.3 +/- 2.7 (P < 0.001). The treatment was well tolerated in most patients. Twelve out of 30 patients reported temporary pain, but only two needed analgesic treatment. Other side-effects included venous bleeding in one patient. The remittance spectra showed an improvement of tissue oxygenation as revealed by the characteristic oxygen doublet peak (548 and 575 nm). Biosurgery is an effective and rapid treatment for the debridement of chronic wounds and the improvement of wound healing. A possible mode of action is the increase in tissue oxygenation. More studies are needed.