267 normal controls of different ages underwent achilles tendon thickness measurements by ultrasonography (US) for reference. 96 recruits and 10 young women additionally underwent magnetic resonance imaging of the achilles tendons and calves for more systematic evaluation of the factors influencing tendon thickness. Children under 10 had a tendon thickness (mean +/- SD) of 4.6 +/- 0.8 mm, 10-17 year-olds 6.1 +/- 0.8 mm, 18-30 year-olds 6.3 +/- 0.5 mm and over 30 year-olds 6.9 +/- 1.0 mm. Women had slightly thinner tendons than men, but the difference was statistically significant only in the oldest age group. Normal variation in shape of the tendon caused up to a 25% variation in the measured thickness values. In the large sample of recruits a statistically significant correlation was found between the tendon thickness and body height. Differences in population height could account for the measured differences in normal achilles tendon thickness found in studies on Japanese subjects compared with studies on European and American subjects.