Ardern, CL, Pizzari, T, Wollin, MR, and Webster, KE. Hamstrings strength imbalance in professional football (soccer) players in Australia. J Strength Cond Res 29(4): 997–1002, 2015—The aim of this study was to describe the isokinetic thigh muscle strength profile of professional male football players in Australia. Concentric (60° and 240°·s−1) and eccentric (30° and 120°·s−1) hamstrings and quadriceps isokinetic strength was measured with a HUMAC NORM dynamometer. The primary variables were bilateral concentric and eccentric hamstring and quadriceps peak torque ratios, concentric hamstring-quadriceps peak torque ratios, and mixed ratios (eccentric hamstring 30°·s−1 ÷ concentric quadriceps 240°·s−1). Hamstring strength imbalance was defined as deficits in any 2 of: bilateral concentric hamstring peak torque ratio <0.86, bilateral eccentric hamstring peak torque ratio <0.86, concentric hamstring-quadriceps ratio <0.47, and mixed ratio <0.80. Fifty-five strength tests involving 42 players were conducted. Ten players (24%) were identified as having hamstring strength imbalance. Athletes with strength imbalance had significantly reduced concentric and eccentric bilateral hamstring peak torque ratios at all angular velocities tested; and reduced eccentric quadriceps peak torque (30°·s−1) in their stance leg, compared with those without strength imbalance. Approximately, 1 in 4 players had preseason hamstring strength imbalance; and all strength deficits were observed in the stance leg. Concentric and eccentric hamstrings strength imbalance may impact in-season football performance and could have implications for the future risk of injury.