Trail Les 40 Bosses

About the Event

In the heart of Île-de-France, nestled within the lush forests of Val-d'Oise, lies an annual trail running spectacle that challenges even the most seasoned athletes. The Trail of the 40 Bumps invites runners to venture into a winter wonderland in December, where elevation gains and scenic beauty await around every corner.
The centerpiece event is the "Monster," stretching over two days with a grueling distance of 120 kilometers. This course features an impressive ascent of 4,900 meters D+, making it one of the toughest trails in the region. As runners traverse through chestnut groves and oak forests under moonlit skies, they will encounter terrain that tests both physical endurance and mental fortitude. The "Monster" is a true test for those who have trained on lesser hills like Montmartre.
For those seeking a more moderate challenge, the 75-kilometer "Beast" offers similar scenic beauty but with an ascent of just 1,900 meters D+. This course still demands significant effort and resilience yet remains accessible to runners looking for a substantial day out in nature. Both events start near Saint-Leu-la-Forêt, where the forest landscapes provide a serene backdrop.
On Sunday, shorter distances cater to all levels: 45 kilometers through historic trails, 20 kilometers offering glimpses of charming villages like Taverny and Saint-Prix, and a more manageable 10-kilometer route for those looking for an introduction. Each course winds its way through dense forests, past quaint towns, and alongside picturesque rivers.
Post-race activities abound in the Val-d'Oise region. History enthusiasts will find much to explore at landmarks such as Chantilly Castle to the north or Écouen Castle with their extensive gardens perfect for a post-run stroll. For art lovers, Auvers-sur-Oise offers an immersive experience where one can trace Van Gogh's final days and artistic inspirations.
The village of Roche-Guyon adds charm with its medieval ambiance, recognized as one of the most beautiful villages in France. Its serene setting provides runners a tranquil spot to unwind after their efforts on the trails.
In December, join thousands at Saint-Leu-la-Forêt for an unforgettable trail running experience that combines breathtaking scenery and challenging terrain, making it the perfect conclusion to your year's training regimen.

Races Offered

Trail 120 km

Dec 05, 2026

straighten

Distance — 120K

terrain

Type — Running

timer

16:30

sell

€110.00

Trail 75 km

Dec 05, 2026

straighten

Distance — 75K

terrain

Type — Running

timer

16:30

sell

€80.00

Trail 45 km

Dec 06, 2026

straighten

Distance — 45K

terrain

Type — Running

timer

08:30

sell

€50.00

Trail 20 km

Dec 06, 2026

straighten

Distance — 20K

terrain

Type — Running

timer

10:00

sell

€30.00

Trail 10 km

Dec 06, 2026

straighten

Distance — 10K

terrain

Type — Running

timer

14:00

sell

€20.00

Weather History

These values represent the 3-year monthly averages for December in Saint-Leu-la-Forêt.

thermostat Temperature

4.2° / 8.8°

wb_sunny Feels like

0.3° / 5.4°

light_mode Avg sunshine hours

2h 47m

water_drop Avg rainfall

1.7 mm

ac_unit Avg snowfall

—

air Avg wind speed

13.5 km/h (Gentle breeze)

opacity Sweat loss rate

Low

terrain Course conditions

Firm and slightly soft

sunny Max UV Index

Low (1.4)

humidity_mid Humidity

88.0%

wb_twilight Sunrise

09:37

nightlight Sunset

17:55

About Saint-Leu-la-Forêt

Learn more about the host city, Saint-Leu-la-Forêt.

Saint-Leu-la-Forêt is a commune in the Val-d'Oise department, in the northwestern outer suburbs of Paris, France. It is located 19.9 km (12.4 mi) from the centre of Paris.