A man suffered a vicious attack by a thug armed with a baseball bat and fire extinguisher.
The incident took place in Kenilworth Rise, Livingston, after a disturbance involving several men on Friday at around 11pm.
The 20-year-old victim was taken to St Johns Hospital with serious injuries.
The main suspect is described as white, of a stocky build and wearing dark clothing and a black balaclava.
Detective Sergeant Adrian Wallis from Police Scotland said: “We are still trying to establish the full circumstances surrounding this assault and at this time we do not know how many individuals were involved, nor do we have any further descriptions.
“As such, we are keen to hear from anyone who was in Kenilworth Rise on Friday evening and remembers seeing anything suspicious.
“In particular, we would like to trace a group of youths who were in the street at the time of the assault and may have witnessed the incident.
“Similarly, anyone with any further information relevant to this investigation is also asked to contact police immediately.”