The purpose of this study was to compare maximal oxygen consumption (VO2max) values (Amax) with predicted VO2max values obtained from the Polar V800 (PV800) and the Garmin Forerunner 230 (GF230) to determine whether the accuracy of these estimates differs between sexes. The PV800 predicts VO2max based on resting heart rate (HR). The GF230 predicts VO2max based on a 10-minute, self-paced outdoor run. Subjects consisted of 22 women and 22 men. After a 10-minute supine rest, predicted VO2max (Pmax) values from the PV800 were measured during a 5-minute supine rest. Subjects then completed a treadmill VO2max test to determine Amax. Within 48 hours of obtaining Amax, individuals completed a 10-minute, self-paced outdoor run using the GF230 to predict VO2max (Gmax). A 2-way (watch vs. sex) analysis of variance was used to determine whether there were differences between predicted VO2max and Amax between sexes. A significant interaction between HR monitors and sex was found (p = 0.007). Follow-up analysis showed that in men, Pmax was significantly overestimated compared with Gmax (3.87 6 1.53 ml·kg-1·min-1, p = 0.013). Bland-Altman plots revealed large random errors that ranged from 6.6 to 16.4 ml·kg-1·min-1 with the PV800 having larger errors than the GF230. The results of this study indicate that the GF230 can provide an accurate estimate of VO2max in both sexes. The PV800 can provide an accurate estimate of VO2max for women but not men. Estimates of VO2max from the both devices should be used with caution because of the large random error associated with them. [ABSTRACT FROM AUTHOR]