Běhej Lesy Jizerská 50 is a summer trail ultramarathon held at the ski stadium in Bedřichov, Jizera Mountains, Czech Republic. The primary race covers 47.4 kilometers with 841 meters of elevation gain, taking place in late August before the area transitions to snow for cross-country skiing. It is part of the Běhej Lesy series, which also includes 12 km and 23 km races for those seeking the mountain environment without the full ultra distance.
The 47.4 km route passes through locations associated with the renowned Jizerská 50 ski race, such as Hřebínek, Smědava, Jizerka, and Olivetská hora. This connection provides the race with a distinct local character, as participants navigate summer trails utilized by both elite and recreational skiers in winter. The participant field is predominantly Czech, with Czech runners frequently securing top positions. Local athlete Přemysl Ježek has won the Jizerská 50 RUN twice. The course is noted for being well-marked and equipped with sufficient aid stations. However, experienced runners advise carrying personal hydration and nutrition, as inadequate fueling can significantly impact performance over the nearly 50-kilometer mountain course.