BACKGROUND Anterior knee pain has been associated with sports activity, especially long-distance running and endurance sports. It is important to determine the incidence of anterior knee pain (AKP) in runners after a half-marathon race and identify possible risk factors associated. METHODS Cross-sectional study where runners from a half marathon race were randomly invited to participate. Participants were recruited at the race kit pickup site the day before the race. Eligible participants completed a survey regarding demographic information, running experience and training details. An orthopedic surgeon performed a physical examination and recorded the medical history. At the finish line, the participants were evaluated again for possible new injuries. RESULTS A total of 205 runners were included in the study, with a 98.5% follow-up rate (n = 203). 24% of runners had an injury at the end of the race (n = 49). Anterior knee pain was the most frequent injury (n = 12), followed by iliotibial band syndrome (n = 10), muscle cramps (n = 7) and hamstring tears (n = 4). Anterior knee pain had a statistically significant association with insufficient stretching of the hamstrings (p = 0.048) and finishing the race in more than 2 h (p = 0.014). CONCLUSIONS Anterior knee pain was the most frequent new injury in the half-marathon runners after the competition. Spending more than 2 h to finish the race and stretching the hamstrings by less than 70° in the supine position were risk factors for anterior knee pain.