Systemic hyperbaric oxygen (HBO) is accomplished when a patient is breathing 100% oxygen in an environment with increased barometric pressure. A typical HBO treatment protocol of diabetic foot ulcer involves 20 to 40 sessions. Treatment is usually given as daily 90- to 120-minute HBO sessions at pressures between 2.0 and 2.5 absolute atmospheres. The wide use of HBO as treatment of diabetic foot ulcers over the past decades has been founded on weak scientific ground (ie, few and small prospective studies with methodologic limitations on top of case series). However, the consistency in positive outcome in these trials evaluating HBO on ulcer healing is noteworthy because these findings are in concert with data from in vitro and physiologic studies supporting the theoretic framework of HBO reversing hypoxia-induced pathology. Two well-designed randomized double-blinded placebo-controlled studies have in recent years put HBO on firmer ground as treatment of a selection of diabetic patients with chronic foot. Some evidence indicates that microvascular parameters such as transcutaneous (partial) oxygen pressure (TcPO(2)) could be useful in predicting which patients will benefit from therapy. Health economic studies suggest potential cost-effectiveness of HBO. But because these analyses are limited by their deficient primary clinical data, they should be interpreted with caution. Thus, HBO is only indicated in a selected group of patients with chronic diabetic foot ulcers. Several key issues remain to be addressed such as developing robust criteria to determine which patients are likely to benefit and when to start and stop treatment.