A number of physiological, anthropometric, and training variables seem to influence running performance, depending upon the length and duration of performance; among these variables, the relation between skinfold thicknesses and running performance has been especially discussed (1-6). More than 20 years ago, Hagan et al reported that apart from other variables, the sum of seven skinfold thicknesses correlated to marathon performance time (5). Bale et al showed that total skinfold thickness, the type and frequency of training, and the number of years running were the best predictors of running performance and success in the 10-km distance (6). In recent studies, an association between the thicknesses of selected skinfolds of the upper and lower body and running performance has been demonstrated in top-class male and female runners who ran distances from 100 m to 10,000 m and the marathon, respectively (1, 2). High correlations were found in male runners between both the front thigh and medial calf skinfold thickness and 10,000-m race times (1). It was supposed that the thickness of skinfolds in the lower limb is a result of intense training in running (2). Ultra-endurance races, defined as an endurance performance of more than 6 hours (8) that can last for days or even weeks (7, 9-11), are increasingly popular. Published reports regarding the association between skinfold thicknesses and performance in ultra-marathon distances are scarce (4). One study including 25 male multistage ultra-marathoners completing ~50 km per day during a 7-day multistage run reproduced the finding that calf skinfold thickness was related to ultra-marathon performance (4). However, no study has investigated whether skinfold thicknesses are related to performance in ultra-marathoners running for more than 50 km without a break. The thickness of skinfolds is related to body fat (12). Since ultra-endurance performance leads to a decrease in body fat (9-11, 13), higher prerace skinfolds might be associated with enhanced ultra-endurance performance. Arrese and Ostariz (1) assumed that the assessment of skinfold values in the lower limbs may be a useful predictor of athletic performance. We therefore investigated correlations between skinfold thicknesses and race performance in male ultra-marathoners in a 24-hour run. We hypothesized that also for male ultra-marathoners completing more than 50 km without a break, an association would exist between skinfold thicknesses of the lower limbs and race performance.