Flap monitoring by oxygen measurement with a microcatheter pO2 probe was carried out in 17 free flaps. The probe represents a new development among monitoring methods, allowing the measurement of oxygen pressure in all types of flaps including buried free flaps and vascularized bone grafts. Fourteen flaps survived with no problem. Three flaps with different degrees of problem are reported, and the advantage, and sources of error using this monitoring method are discussed.