To validate VO2 and energy expenditure predictions by the Suunto heart rate (HR) system against a first principle gas analysis system, well-trained male (n = 10, age 29.8 +/- 4.3 years, VO2 65.9 +/- 9.7 ml x kg x min) and female (n = 7, 25.6 +/- 3.6 years, 57.0 +/- 4.2 ml x kg x min) runners completed a 2-stage incremental running test to establish submaximal and maximal oxygen uptake values. Metabolic cart values were used as the criterion measure of VO2 and energy expenditure (kJ) and compared with the predicted values from the Suunto software. The 3 levels of software analysis for the Suunto system were basic personal information (BI), BI + measured maximal HR (BIhr), and BIhr + measured VO2 (BIhr + v). Comparisons were analyzed using linear regression to determine the standard error of the estimate (SEE). Eight subjects repeated the trial within 7 days to determine reliability (typical error [TE]). The SEEs for oxygen consumption via BI, BIhr, and BIhr + v were 2.6, 2.8, and 2.6 ml.kg.min, respectively, with corresponding percent coefficient of variation (%CV) of 6.0, 6.5, and 6.0. The bias compared with the criterion VO2 decreased from -6.3 for BI, -2.5 for BIhr, to -0.9% for BIhr + v. The SEE of energy expenditure improved from BI (6.74 kJ) to BIhr (6.56) and BIhr + v (6.14) with corresponding %CV of 13.6, 12.2, and 12.7. The TE values for VO2 were approximately 0.60 ml x kg x min and approximately 2 kJ for energy expenditure. The %CV for VO2 and energy expenditure was approximately 1 to 4%. Although reliable, basic HR-based estimations of VO2 and energy expenditure from the Suunto system underestimated VO2 and energy expenditure by approximately 6 and 13%, respectively. However, estimation can be improved when maximal HR and VO2 values are added to the software analysis.