The influence of body mass on cross-country skiing performance was evaluated by (i) a theoretical analysis of the relationship between body mass (M) and on one hand the maximal aerobic power and on the other the power expense of skiing, (ii) timing skiers in varied terrain during ski racing, (iii) comparing racing success between light and heavy skiers, and (iv) relating maximal aerobic power of skiers and power expense of skiing to body mass. The theoretical analysis indicated that the maximal aerobic power scaled with M2/3, while the mass exponent for the power expenses ranged from 1/3 to 1. The net result was that heavy skiers appear to be favored in all types of terrain but the steep uphills. During ski racing, heavy skiers displayed a tendency to be faster in all types of terrain, although no statistically significant effect was found. The racing success tended to be greater in heavy skiers among males but not among females. During skiing, maximal aerobic power and the power expense of skiing scaled with M0.7 and M0.4, respectively, indicating an advantage for the heavy skier. Altogether, these observations indicate that heavy skiers are favored, but also that other factors are more important than body mass for cross-country ski racing performance.