It is speculated that anaerobic metabolism is the predominant source of energy in karate kumite. However, no experimental proof is currently available. The metabolic cost and fractions of aerobic and anaerobic energy of karate kumite fighting were investigated. Ten male nationally or internationally ranked karateka [means (SD) age 26.9 (3.8) years, height 1.80 (0.08) m, mass 77.2 (12.8) kg] performed two to four fights scheduled and judged like a championship. Oxygen uptake was measured continuously with a portable spirometric device. Blood lactate was determined immediately before, and minute by minute after, each fight. Aerobic, anaerobic alactic and anaerobic lactic energy were calculated from oxygen uptake during the fight ( VO(2)), the fast component of the post-fight oxygen uptake ( VO(2PCr)) above resting values and changes in blood lactate concentration (Net-BLC), respectively. Altogether, 36 fights lasting 267 (61) s were analysed. The referee's decisions caused an activity-to-break ratio of approximately 2:1. VO(2), VO(2PCr), and Net-BLC per fight were 165.3 (52.4) ml(.)kg(-1), 32.2 (7.2) ml(.)kg(-1)and 4.2 (1.9) mmol(.)l(-1); the overall energy cost above rest was 334.3 (86.3) kJ per fight. Fractions of aerobic, anaerobic alactic, and lactic energy sources were 77.8 (5.8)%, 16.0 (4.6)%, and 6.2 (2.4)%, respectively. The results indicate a high metabolic rate in karate kumite. However, the acyclic activity profile implies that aerobic metabolism is the predominant source of energy and there is anaerobic supplementation, mainly by high-energy phosphates.