Warwickshire Police attended the Elephant and Castle in Emscote Road following a report of an altercation between two men in the smoking area at the front of the premises at approximately 11.45pm.
A man, aged 30, from Warwick, was unconscious and was taken to University Hospitals Coventry and Warwickshire by ambulance. Tonight he remains in hospital.
He has suffered a serious head injury and is said to be in a serious condition.
A 20 year-old man, from Warwick, has been arrested on suspicion of grievous bodily harm in connection to the incident and currently remains in police custody.
Officers would like to speak to anyone who witnessed the altercation or who saw a black Golf and silver Vauxhall Vectra leaving the area between 11:45pm and midnight on Friday.
Anyone who has any information can contact Warwickshire Police on 101, quoting incident number 446 of March 27, 2015.
Alternatively, information can be given anonymously to the independent charity, Crimestoppers, on 0800 555 111 or via www.crimestoppers-uk.org