This study assessed the relationship between haemoglobin mass (Hbmass) and maximum oxygen consumption (VO2max) 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). Hbmass and VO2max were measured approximately every 3 months. At baseline there was a high correlation (r = 0.82, P< 0.0001) between relative VO2max (ml kg-1 min-1) and relative Hbmass (g kg-1). During 12 months there was a significant increase in relative VO2max of the cyclists but not the controls; however, there was no corresponding increase in relative Hbmass of either group. The correlation between percent changes in relative VO2max and relative Hbmass was not significant for cyclists (r = 0.31, P = 0.33) or controls (r = 0.42, P = 0.19). Training does not increase relative Hbmass in adolescents consistent with a strong hereditary role for Hbmass and VO2max. Hbmass may be used to identify adolescents who have a high VO2max.