Two men have been hospitalised following an unprovoked headbutt attack at Sydney's The Star casino overnight.

A man who was refused entry to Sydney's The Star casino allegedly headbutted a random stranger before being chased by a group of the assaulted man's friends, in an incident that left two men at St Vincent's hospital in a stable condition, police say.

Police were called to The Star casino in Pyrmont about 1.25 am on Saturday, after reports of the incident.

Police investigators were told that two men were in the The Star casino complex, but were refused entry into the casino itself on account of their drunkenness, the two men then walked through the complex food court, where one of them headbutted a stranger.

The reason for the headbutt is unknown, but high levels of intoxication appear to be a factor, Police say.