Breeding success among male white-tailed deer (Odocoileus virginianus) has been correlated with numerous physical and physiological variables. We investigated the effect of relative body mass on breeding success in captive male deer from 21 breeding trials. Contrary to our prediction, larger males on average had identical levels of breeding success (59% of breeding opportunities) in the 1st and 2nd halves of the breeding season. The variability of breeding success increased with greater relative mass; thus deer with substantial mass disadvantages had only limited success, and males weighing ≥92% of the pen average had success ranging from 0% to 100% of potential breeding opportunities. Although constant proximity limited males primarily to a strategy of direct confrontation, some subordinate males evidently mated opportunistically when >1 female was receptive. We conclude that ≥1 uncontrolled variable, possibly related to aggressiveness or life-history strategy, remained influential and limited the ability of some males to compete successfully in spite of greater relative body mass.