The purpose of this study was to determine the variables that most limit middle distance run performance e.g. 800, 1500 and 3000 meters. A predictive research design was employed as 56 runners ran each of the three distances and were evaluated for VO2max, running mechanics, physique variables, ventilatory threshold, and anaerobic capacity and power. The data were analyzed by correlation procedures and stepwise regression analysis. The variables that most limited 1500 and 3000 meter (m) run performance were identical. Maximal oxygen uptake was the most important, but stride length and anaerobic capacity and power also contributed significantly. Run performance at 800 m was influenced by a considerably different set of variables. Peak velocity was most important, but VO2max, % fat and thigh length also contributed significantly. These data indicate that 1500 and 3000 m runners can compete favorably at either distance, while 800 m runners have a different profile and would experience difficulty competing at 1500 and 3000 meters.