This study examined the physical demands experienced by top-class assistant referees during high-standard football matches. Computerized match analyses and heart rate recordings were performed on 18 international assistant referees during the FIFA Confederations Cup 2005. Total distance covered averaged 5752+/-554 m, of which 34% was covered at a high-intensity (>13 km.h (-1)). Sidewards movements represented 30% of total match distance. Both total distance (r=0.67; P<0.01) and high-intensity activities (r=0.52; P<0.05) were related to the displacements of the ball in the same match. The distance covered by high-speed running in this tournament was 255% greater (P<0.001) than that covered during the under-17 World Championship 2003. Mean heart rate during the game was 140+/-11 b.min (-1) (78+/-4% HR (max)). Mean heart rate during a 5-min interval of the match was related in part (r=0.31; P<0.001) to the amount of high-intensity activities performed within the same match-period. The results of this study show that the physical demands imposed on assistant referees are influenced by the movements of the ball and the standard of the competition.