Police were called by London Ambulance Service at 8.08pm on Monday, 14 September to Pield Heath Road near the junction with Copperfield Avenue, Uxbridge.
Officers attended the location where a 33-year-old man was found suffering an abdominal injury caused by a crossbow arrow.
He has been taken to a west London hospital, where his condition is being treated as life-threatening.
There have been no arrests and enquiries continue.
Cordons remain in place at the scene.
Witnesses, or anyone with information, should call police on 101 quoting CAD 7227/14Sep. To remain anonymous, please contact Crimestoppers.
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