Endurance trained athletes are reported to have higher total blood volume and hemoglobin mass (Hb-mass) than untrained individuals. It is unknown if a short period of endurance training increase total blood volume and Hb-mass in inactive obese subjects. The primary aim of the study was to investigate the effect of 6 weeks of high intensity interval training (HIIT) (4×4 and 10×1 minutes at 90-95% of Maximal Heart rate (HRmax) and moderate continuous training (MCT) at 75 % HRmax on total blood- and plasma volume and Hb-mass in inactive obese subjects. Thirty obese inactive men and women were randomized 1:1:1 to 3 training groups and supervised for 6 weeks with 3 weekly treadmill training sessions. Training was performed as either 1) 4 x 4 minutes HIIT. 2) 10 x 1 minutes of HIIT 3) 47 minutes of MCT. The improved carbon monoxide-rebreathing method was used to measure total blood volume and Hb-mass before and after the training intervention. Measurements were done in the morning in the fasted state, with 500 ml of water consumed minimum 1 hour before testing. Maximal oxygen uptake test, a performance test, measurement of endothelial function was performed, and blood and muscle biopsy samples collected before and after training intervention. Twenty-one subjects completed the training intervention with a complete blood volume measurement, a total of 6, 7 and 8 subjects in the 4×4, 10×1 and MCT group respectively (age 41 ± 9 years, height 173 ± 8 cm, weight 91 ± 11 kg, BMI 30 ± 4, VO2max 35.79 ± 7.25). At baseline, total blood volume was 5.80 ± 0.62, 5.68 ± 1.07 and 5.95 ± 1.22 L in the 4×4, 10×1 and MCT groups respectively, with no significant change after training. At baseline, total plasma volume was 3.53 ± 0.34, 3.50 ± 0.61 and 3.64 ± 0.72 in the 4×4, 10×1 and MCT groups respectively. After training total plasma volume was increased by 4% to 3.65 ± 0.50 L (p ≤ 0.05) in the 10 x 1 minute interval training group with no change in the other groups. Baseline total Hb-mass was 758 ± 126, 729 ± 759 and 788 ± 192 grams in the 4×4, 10×1 and MCT groups respectively. After training the 10×1 group increased total Hb-mass by 4 % to 760 ± 155 g (p ≤ 0.05), with no change in the other groups. When values are reported per kg body weight, blood volume, plasma volume and Hb-mass was significantly increase from before to after training in the 10×1 group (p ≤ 0.05), with no change in any of the other groups. Body weight remained unchanged from before to after training in all group. These data suggest that 6 weeks of 10×1 minutes HIIT significantly increase plasma volume and total Hb-mass in obese untrained subjects.