Ten kilometer performance and predicted velocity at VO2max among well-trained male runners.Med. Sci. Sports Exerc, oL 21, No. 1, pp. 78-83, 1989. Previous research (study 1) has shown that a significant relationship exists between 10 km run time (RT) and predicted running velocity at VO2max(vVO2max) among welltrained males heterogeneous in VO2max. Since competitive runners often display a homogeneous fitness profile, the purpose of this study was to determine the association between 10 km RT and vYO2maxamong a group of trained runners exhibiting nearly identical VO2maxvalues (study 2). Running economy (RE), vVO2max, and velocity at a 4 mM blood lactate concentration (v at 4 mM BL) were calculated in both studies. Correlations were obtained as shown in Table 2. The relationship between VO2max and 10 km RT achieved statistical significance only in study 1, while RE explained a greater amount of performance variation in study 2. In both studies, variation in 10 km RT attributable to vVO2maxwas similar and exceeded that due to either VO2max or RE. vVO2maxalso accounted for essentially the same amount of variation in 10 km RT as did v at 4 mM BL. It was concluded that, among well-trained subjects homogeneous in VO2max, a strong relationship exists between 10 km RT and vVO2maxthat appears to be mediated to a large extent by RE.