In prepubertal stage, total hemoglobin mass (Hbmass) is similar in boys and girls. With the beginning of puberty, Hbmass increases in boys, while it stagnates in girls. This divergance might be explained by the selective increase in androgen production in males which has, however, not yet been studied. PURPOSE: To study the relationship between the blood testosterone concentration and hemoglobin mass in boys and girls. A second aim was to find out the impact of altitude and training status. METHODS: In total, 313 children and adolescents with different endurance training status and different altitude of residence entered the study (age 9 to 18 yrs; females n=94, males n=219; low altitude (1000m) n= 150, moderate altitude (2500m-3000m) n= 163; endurance trained n=190, untrained n=123) . The sexual maturation was estimated by the scale of Tanner (stage I-V). Hbmass and blood volume (BV) were determined by the optimized CO-rebreathing method. Testosterone (Test) and erythropoietin (EPO) were measured in cubital venous blood. For statistical purposes multiple linear regression analyses and regression trees (CART) were used. RESULTS: While [Test] was low in all females (0.2 ±0.6ng/ml) it markedly increased from Tanner stage III (stage I/II: 0.4 ±0.8ng/ml, stage III: 2.7 ±2.0ng/ml, stage IV/V: 4.2 ±2.1ng/ml). In the whole group (males and females) a strong correlation was found between [Test] and absolute Hbmass (r = 0.8, p <0.001) as well as relative Hbmass (Hbmass/kg, r = 0.6, p <0.001). In the male group, the increase in [Test] by 1 ng/mL was associated with an increase of 34.2 ±4.3g of Hbmass (p <0.001), the training status with 45.8g and altitude with 32.1g. Highest Hbmass values (15.3 ±0.7g/kg) were observed in the trained group from altitude with [Test] > 4.9ng/ml). The general impact of [Test] on Hbmass was 45%, of biological maturation 34.4%, of training 16.0%, and of altitude 5.0%. A strong correlation of [Test) with BV was found (r = 0.7, p <0.001). There was no correlation between [Test] and EPO (r = 0.04, p= 0.5). CONCLUSION: In boys there is a strong increase in [Test] from Tanner stage III, which considerably stimulates erythropoietic processes. We suggest testosterone being the determining factor for the different development of Hbmass in girls and boys during puberty and explaining the gender differences in adults.