Exercise is normally recommended for its beneficial effects on health; however, exercising at very high heart rate (HR) may increase the risk of cardiac events. Theoretical maximal HR (TMHR) is subject dependent and may be easily computed through several formulas; according to the guide-lines, recommended target HR range during a stress test for clinical evalua-tion is 50 to 85% of TMHR. Considering the new widely diffused use of wearable sensors, an into-the-field assessment of the highest HR (HHR) reached during exercise is now possible. Thus, the present study aims to as-sess HHR during uncontrolled exercise and to relate it to TMHR. To this aim, 178 HR series were acquired through the BioHarness 3.0 by Zephyr from 122 athletes, while practicing 15 different sports (Aerial Skills, American Football, Basketball, CrossFit, Cycling, Fitness, Jogging, Middle Distance Running, Running, Skating, Soccer, Zumba, Tennis, Trekking). TMHR was assessed by applying the following formulae: TMHRG=206-0.88∙age; TMHRI=205.8-0.685∙age; TMHRK=220-age; TMHRL=206.3-0.711∙age; TMHRM=217-0.85∙age; TMHRN=211-0.64∙age; TMHRT=208-0.7∙age. Percentages of athletes whose HHR overcame 85% of TMHR and TMHR were computed. Moreover, HHR and TMHR distributions were compared by paired T-Student test (statistical significance at 0.05). HHR of 90% or more of athletes overcame 85% of TMHR (specifically, HHR of 94%, 93%, 91%, 90%, 90%, 90%, and 92% of athletes overcame 85% of TMHRG, TMHRI, TMHRK, TMHRL, TMHRM, TMHRN and TMHRT, respectively). HHR of 39% or more of athletes overcame TMHR (HHR of 66%, 53%, 39%, 55%, 39%, 39%, and 48% of athletes overcame TMHRG, TMHRI, TMHRK, TMHRL, TMHRM, TMHRN and TMHRT, respectively). HHR (191±26 bpm) was significantly higher than TMHRG (180±10 bpm; P<0.01), TMHRI (186±8 bpm; P<0.05) and TMHRL (186±8 bpm; P<0.01) but not significantly different from TMHRK (191±12 bpm), TMHRM (192±10 bpm), TMHRN (192±7 bpm), and TMHRT (188±8 bpm). Thus, HHR normally approaches, and sometimes overcomes, TMHR when exercising.