Trail Les 40 Bosses 2026

Trail Les 40 Bosses is a December trail-running weekend held in Saint-Leu-la-Forêt, Val-d’Oise, on Paris's northern edge. Races begin and end near Rue du Général Leclerc by Gymnase Les Dourdains, leading runners into the Montmorency forest. Saint-Leu-Outdoor organizes the event with support from the Red Cross and volunteers, offering forest races from 10 km and 20 km up to the 120 km “Le Monstre.” This 100M UTMB qualifier features approximately five thousand metres of elevation gain. The courses are hilly for Île-de-France, with the longest race comprising two large loops: a 75 km loop across the massif, incorporating new trail sections, followed by the historic 45 km route. Runners will pass landmarks like the Tour du Plumet, Pont du Diable, and Étangs Marie, alongside secluded single-track trails near Béthemont, Bessancourt, and Chauvry. The forest terrain includes chestnut, oak, beech, and conifers, and the December timing suggests the possibility of cold weather or snow.