Mountain marathons have been developed rapidly the last years in Greece, as a leisure activity. The aim of the present study was to investigate constraints that influence participation in Olympus Marathon. One hundred and one runners (95% male, 5% female) participated in the study. A modified version of Alexandris & Carroll’s (1997a) leisure constraints questionnaire was used. The results revealed four constrain dimensions: lack of time, individual / psychological problems, financial/accessibility and facilities/ services related problems. Furthermore, the results revealed statistical significant differences between males and females (p<0.05) in the financial/accessibility dimension, and between the age groups of 31-40 and 41-50 in the time dimension (p<0.01). The study’s results indicate the value of leisure constraints research in studying athletes’ behavior in the context of mountain marathons, and can help organizers’ efforts to increase athlete’s participation.