It has been shown earlier that the proportion of slow twitch (type I) fibres in the leg musculature is directly related to running performance in boys but not in girls at the age of 16. The purpose of the present study was to determine whether this pattern remains in adulthood. Forty-one men and 19 women were tested at the age of 16 and 27 years. Muscle biopsy specimens were obtained from the vastus lateralis and analysed for fibre type (I%, IIA%, IIB%). Maximal oxygen uptake (VO2max) was estimated from a submaximal exercise test. Running performance (9-min run) was assessed in a modified Cooper test. At age 16, the 9-min run was positively correlated with type I% and VO2max for the men. For the women, the 9-min run was positively correlated with VO2max but not with type I%. At age 27, the 9-min run was positively correlated with type I % and VO2max for both men and women. The appearance at the age of 27 of a relationship between running performance and type I% in the women could be related to the increase with age in the VO2max found in the women in the present study. These results support an earlier formulated hypothesis that a certain level of VO2max must be reached before peripheral factors such as the muscle fibre type composition are of any significance for the performance in a 9-min run.