McClelland, JM, Godek, SF, Chlad, PS, Feairheller, DL, and Morrison, KE. Effects of cardiovascular fitness and body composition on maximal core temperature in collegiate football players during preseason. J Strength Cond Res 32(6): 1662–1670, 2018—This study evaluated the effects of body mass index (BMI) and aerobic fitness (V[Combining Dot Above]O2max) on maximal core temperature values (Tcmax) in 17 National Collegiate Athletic Association Division III football players during preseason. The subjects included 9 backs (BKs) and 8 linemen (LM). V[Combining Dot Above]O2max testing was performed 1 week before preseason. Core temperature was monitored by ingestible sensor every 10 minutes during practices on day 4 (D1), day 5 (D2), day 7 (D3), and postacclimatization on day 14 (D4). Wet bulb globe temperature (WBGT) was recorded on each collection day. Independent, paired t-tests and Pearson's correlations were performed (α = 0.05). There were no significant correlations between V[Combining Dot Above]O2max and Tcmax on D1 (WBGT = 29.07° C) or D2 (WBGT = 30.93° C), but on D3 (WBGT = 31.39° C) there was a nonsignificant moderate negative correlation (r = −0.564, p = 0.090). There were no significant correlations between BMI and Tcmax on D1 or D2, but on D3 there was a nonsignificant moderate positive correlation (r = 0.596, p = 0.069). Paired t-tests revealed that overall Tcmax (D1–3) (38.56 ± 0.32° C) was statistically higher (p = 0.002) than D4 (38.16 ± 0.30° C). Independent t-tests between groups showed that the Tcmax values during preacclimatization (D1–D3) were significantly higher in LM (38.50 ± 0.37° C) than BKs (38.16 ± 0.35° C) (p = 0.007). V[Combining Dot Above]O2max was significantly lower (p = 0.006) in LM (36.89 ± 6.40 ml·kg−1·min−1) than BKs (47.44 ± 7.09 ml·kg−1·min−1), and BMI was significantly higher (p = 0.019) in LM (35.59 ± 4.00 kg·m−2) than BKs (28.68 ± 3.38 kg·m−2). The results of this study demonstrate that LM are significantly less fit than BKs and have a greater BMI. When WBGT was the highest on D3, the results suggest that those with lower V[Combining Dot Above]O2max and higher BMI experienced a higher Tcmax.