['To investigate whether adipose subcutaneous tissue or skeletal muscle mass decreased during a non-stop ultra-endurance triathlon.', 'Descriptive field study.', 'The Triple Iron Triathlon Germany 2006 in Lensahn: 11.6 km swimming, 540 km cycling and 126.6 km running.', '17 male Caucasian triathletes, mean (SD) age 39.2 (7.5) years, height 178 (5) cm, body mass 80.7 (8.9) kg and body mass index (BMI) 25.4 (2.4) kg/m(2).', 'None.', 'Determination of body mass, skin-fold thicknesses, limb circumference, skeletal muscle mass and percentage body fat in order to show changes after the race.', 'A significant decrease was shown for body mass (p<0.001), BMI (p<0.001) and calculated percentage body fat (p<0.001) whereas skeletal muscle mass did not change significantly (p>0.05). Circumferences of the thigh, upper arm and calf did not decrease significantly (p>0.05), whereas all skin-fold thicknesses decreased significantly (p<0.05), with the exception of those at the chest and thigh. A significant correlation was found between the loss of percentage body fat and the loss of body mass (p<0.01, r(2) = 0.55) as well as change in percentage body fat with race performance (p<0.05, r(2) = 0.24).'] Is decrease in body fat during an ultra-endurance triathlon associated with race intensity?