Trexler, ET, Smith-Ryan, AE, Blue, MNM, Schumacher, RM, Mayhew, JL, Mann, JB, Ivey, PA, Hirsch, KR, and Mock, MG. Fat-free mass index in NCAA Division I and II collegiate American football players. J Strength Cond Res 31(10): 2719–2727, 2017—Fat-free mass index (FFMI) is a height-adjusted assessment of fat-free mass (FFM), with previous research suggesting a natural upper limit of 25 kg·m−2 in resistance trained male athletes. The current study evaluated upper limits for FFMI in collegiate American football players (n = 235) and evaluated differences between positions, divisions, and age groups. The sample consisted of 2 National Collegiate Athletic Association Division I teams (n = 78, n = 69) and 1 Division II team (n = 88). Body composition was assessed via dual-energy x-ray absorptiometry and used to calculate FFMI; linear regression was used to normalize values to a height of 180 cm. Sixty-two participants (26.4%) had height-adjusted FFMI values above 25 kg·m−2 (mean = 23.7 ± 2.1 kg·m−2; 97.5th percentile = 28.1 kg·m−2). Differences were observed among position groups (p < 0.001; η2 = 0.25), with highest values observed in offensive linemen (OL) and defensive linemen (DL) and lowest values observed in offensive and defensive backs. Fat-free mass index was higher in Division I teams than Division II team (24.3 ± 1.8 kg·m−2 vs. 23.4 ± 1.8 kg·m−2; p < 0.001; d = 0.49). Fat-free mass index did not differ between age groups. Upper limit estimations for FFMI seem to vary by position; although the 97.5th percentile (28.1 kg·m−2) may represent a more suitable upper limit for the college football population as a whole, this value was exceeded by 6 linemen (3 OL and 3 DL), with a maximal observed value of 31.7 kg·m−2. Football practitioners may use FFMI to evaluate an individual's capacity for additional FFM accretion, suitability for a specific position, potential for switching positions, and overall recruiting assessment.