Hemoglobin mass (Hbmass) is an important factor for endurance performance and athletes born and living at altitude are described to possess elevated Hbmass values due to lifelong hypoxic exposure. It may be hypothesized that this adaptation already occurs in early life. To date, however, there exists only few data on the training influences on Hbmass during childhood and no data exists describing chronic hypoxic effects in children and adolescents. PURPOSE: To investigate the effects of age, sex, training status and altitude on Hbmass in children and adolescents in a cross-sectional study. METHODS: 436 children, 197 females and 237 males, homogenously distributed over the age from 9 to 18 years participated in the study. 189 of them were born and lived permanently at 960m, 247 at 2600m. 168 did not practice any training and 268 were endurance trained with a mean endurance training volume of 14.0h ±5.8/week. Hbmass was determined using the optimized CO-rebreathing technique and VO2max by an incremental step-test on a treadmill. Analyses of variance and multiple regression analyses were performed to estimate the effects of sex, body mass, age, sexual development (scale according to Tanner), training status, and altitude on Hbmass. RESULTS: Overall, ANOVA yielded highly significant effects for sex, age, body mass, altitude and training state (all p< 0.001); and regression analysis (r=0.91) showed highest effects of sex (+121.5g, ß=0.31 in males) and body mass (9.5g/kg, ß=0.57). Because Hbmass of girls reached a plateau at approx. 14 years and boys increased Hbmass until the age of 17 regression analyses were performed separately for females and males and the following effects were found: Males: r=0.93; Hbmass = 11.8*kg + 15.4*years + 60.9 (for trained status) + 35.1 (for altitude residents) + 13.7*(stage of Tanner) - 254.2; females: r=0.84; Hbmass = 6.8*kg + 9.8*years + 54.6 (trained) + 36.9 (altitude) -24.8. Absolute and relative VO2max was closely related to Hbmass (absolute values: r=0.85, y=3.57x + 308; relative values: r=0.70, y=3.56x+6.1). CONCLUSION: Beside the well-known effects of body mass, age and sex also growing-up at altitude and endurance training have remarkable effects on Hbmass in children and adolescents. VO2max is closely related to Hbmass. Like in adults, a change by 1 gram changes VO2max by 3.5ml/min.