The present study examined the cross-sectional area (CSA) of the Achilles tendon in subjects who repeatedly exposed their tendons to large loads (habitual runners) compared to control subjects (non-runners). Six male habitual runners [36 (7) years, 70.9 (4.4) kg and 1.84 (0.05) m, +/-SD] who had performed distance running (approximately 80 km per week) for the last 5 years were compared to six non-runners [34 (3) years, 81.2 (8.7) kg and 1.81 (0.02) m, +/-SD]. Tendon CSA was obtained from MR images obtained with the ankle in a neutral position (90 degrees ). The most proximal aspect of the tuberosity of calcaneus was used as a landmark to standardize the levels of images: the most distal image (1) was obtained 10 mm above the proximal tuberosity of calcaneus, and the most proximal image (7) was obtained 70 mm above the proximal tuberosity of calcaneus. There was a significant difference in CSA along the length of the tendon both in runners ( P<0.001) and non-runners ( P<0.01). In non-runners and runners the CSA of the most distal part was 51% and 85% greater than the most proximal part of the tendon, respectively. Furthermore, there was a difference in tendon CSA between the groups, such that runners had a greater CSA (36%) than non-runners at the most distal part of the tendon ( P<0.05). The greater CSA in the distal tendon may reflect differences in structural properties along the length of the human Achilles tendon, while the greater CSA in runners compared to non-runners may indicate a region-specific hypertrophy in response to the habitual loading of running.