Attempts were made to clarify whether or not fiber composition of the m. vastus lateralis could be predicted with running performance. Biopsy samples from 32 well-trained and 17 untrained adult males were examined for the percentage area of fast-twitch (FT) and slow-twitch (ST) fibers which might be related to the physical performance better than the fiber type distribution. In addition, each subject completed 50-m sprint and 12-min run tests. A multiple regression analysis revealed that in the trained males predictive accuracy (R^2) for the percentage area of FT fibers (% areaFT) from the ratio of a 50-m sprint speed to a 12-min run speed (50 m・S/12 min・S) was higher than that from most of other variables,e.g. 50 m・S, 12 min・S, or the combination of 50 m・S and 12 min・S; R^2 of 50 m・S/12 min・S was 80.3%(p<0.05). A positive correlation between 50 m・S/12 min・S and %areaFT also existed for the untrained subjects (R^2=63.7%, p<0.05). The linear regression equations of %areaFT (Y) on 50 m・S/12 min・S (X)were Y=-68.6 + 76.2X (r=0.896, p<0.05) and Y=-47.5 + 61.1X (r=0.798, p<0.05) for the trained and untrained males, respectively. There was no significant difference in the regression equations between the trained and untrained males. For all subjects, the equation was Y=-59.8 + 69.8X (r=0.876, p<0.05) and the standard error of estimate of %areaFT on 50 m・S/12 min . S was 8.86%. These results suggest that the performances of a 50-m sprint and a 12-min run are valuable indicators in, accurately, easily and noninvasively, predicting the percentage area of FT and ST fibers of the m. vastus lateralis from adult male.