The relationship between magnetic resonance (MR) relaxation times and muscle fiber composition was investigated in 16 men, and a high positive correlation was found. Higher proportions of fast-twitch fiber were associated with longer relaxation times, indicating that fast-twitch fiber has a longer relaxation time than slow-twitch fiber. Multiple regression analysis, conducted to investigate the suitability of relaxation time as a model of estimating muscle fiber composition, revealed that T1 and T2 were significantly related to muscle fiber composition. These results indicate that MR relaxation time may be used to estimate muscle fiber composition.