One person was rescued and other patients evacuated from their beds after fire broke out at the Conquest hospital in Hastings.
Fire crews were called at 12.30am on Sunday morning after the blaze started at the mental health centre.
There are no reports of any injuries.
A spokesman for East Sussex Fire and Rescue Service said four pumps were called, and the fire was put out by 2am.
He added that crews returned to the scene this morning to check for damage.
Investigations are under way to establish how it started.
There are renewed calls for health bosses to re-consider a decision to downgrade a maternity department in Sussex. The unit at Eastbourne District Hospital will be shut temporarily and replaced with a birth centre.
Campaigners fear it could become permanent. It means some expectant mothers will have to travel to the Conquest in Hastings to give birth.
The Lib Dem MP for Eastbourne, Stephen Lloyd told the house of Commons the plans were 'absolutely outrageous'
Controversial plans to change health services in Sussex have been debated in the House of Commons.
MP Stephen Lloyd is trying to overturn a decision to centralise some hospital departments, which has sparked large demonstrations.
Stroke care is to be sited at Eastbourne District Hospital with some emergency surgery at the Conquest in Hastings. We spoke to Eastbourne MP Stephen Lloyd earlier.
The NHS Sussex board has agreed to East Sussex Healthcare NHS Trust plans to centralise some services at its hospitals.
It means services will be split across Eastbourne District General Hospital and The Conquest Hospital in St Leonards.
Stroke care will now be based solely at the Eastbourne hospital, while emergency orthopaedics and emergency general surgery will only take place at the Conquest.
At the moment all the above services are available at both sites. Changes will take effect in about a years time.
A decision is likely to be made today on the future of hospitals in East Sussex.
A public meeting will be held tonight to discuss controversial changes to Eastbourne District General Hospital (DGH) and the Conquest Hospital in St Leonards, but it's thought the NHS Sussex board will decide this lunchtime how it will centralise services.
East Sussex Healthcare NHS Trust is proposing changes that will mean urgent and high risk orthopaedic surgery will no longer be carried out in Eastbourne. Critically ill patients will instead have to travel to St Leonards.
Stroke services will also be centralised at the DGH.
All three services are currently available at both hospitals but health bosses say they must be based on just one site to sustain them and provide expert care.
Campaigners say lives will be put at risk and the changes will put other services like maternity under threat.
The public meeting at Eastbourne Town Hall is being organised by the Save the DGH campaign and is supported by the town's MP Stephen Lloyd.
Click here for reaction from campaign leader Liz Walker, Eastbourne Lib Dem MP Stephen Lloyd and leading GP Martin Writer.
Health chiefs want to concentrate stroke, general surgery and orthopaedic services at either The Conquest in Hastings or Eastbourne District General. They say it will speed up treatment of patients. But campaigners fear it could reduce Eastbourne to little more than a 'cottage hospital'.