The man, in his 30s, was found just after 05:00 on Wednesday lying on the ground between Talisman Rise and Templar Rise in Dedridge, Livingston.
He had sustained a "significant" head injury and was taken by ambulance to St John's Hospital in Livingston.
Police Scotland said the man remains in a stable condition.
DC Iain Duncan, of Police Scotland, said: "At this stage of inquiry we do not know what has happened to this man and we are now working to ascertain the full set of circumstances.
"Today, I urge anyone who was in the area between midnight and 6am on Wednesday and who has information relevant to our investigation to please come forward immediately."
Police are appealing for witnesses after a man was found with a serious head injury on a path in West Lothian.