This study assessed the relationship between haemoglobin mass (Hb(mass)) and maximum oxygen consumption (VO(2max)) in adolescents over 1 year. Twenty-three subjects (11-15 years) participated; 12 undertook ~12 months of cycle training (cyclists) and 11 were sedentary (controls). Hb(mass) and VO(2max) were measured approximately every 3 months. At baseline there was a high correlation (r = 0.82, P < 0.0001) between relative VO(2max) (ml kg(-1) min(-1)) and relative Hb(mass) (g kg(-1)). During 12 months there was a significant increase in relative VO(2max) of the cyclists but not the controls; however, there was no corresponding increase in relative Hb(mass) of either group. The correlation between percent changes in relative VO(2max) and relative Hb(mass) was not significant for cyclists (r = 0.31, P = 0.33) or controls (r = 0.42, P = 0.19). Training does not increase relative Hb(mass) in adolescents consistent with a strong hereditary role for Hb(mass) and VO(2max). Hb(mass) may be used to identify adolescents who have a high VO(2max).