The purpose of this study was to assess the validity of the FitMate metabolic system (Cosmed, Rome, Italy) in measuring oxygen consumption during graded exercise. The FitMate is a new, small (20 x 24 cm) metabolic analyzer designed for measurement of oxygen consumption during rest and exercise. Subjects included 40 healthy adults (N = 20 males, N = 20 females) ranging in age from 18 to 37 kg/m2 (mean +/- SD age, 22.5 +/- 3.6 years) and body mass index (BMI) from 18.3 to 32.5 kg/m2 (23.2 +/- 3.3 years). One-minute FitMate and Douglas bag measurements were made during steady state conditions at the end of each 3-minute stage of the Bruce treadmill graded exercise test, and subjects continued until they could not attain steady state exercise during a stage. Oxygen consumption difference scores (Douglas bag minus FitMate measurements) did not differ between males and females, so data were combined and analyzed for the entire group. During the first three stages, mean oxygen consumption did not differ significantly between the Douglas bag and FitMate systems (26.5 +/- 1.1 and 26.7 +/- 1.3 ml.kg-1.min-1, respectively, P = 0.140) with a mean absolute difference of 0.23 +/- 0.91 ml.kg-1.min-1 or 14.2 +/- 67.5 ml.min-1. In conclusion, the FitMate metabolic system accurately measures oxygen consumption during graded treadmill exercise when compared with the Douglas bag system in male and female adults.