Police are investigating the death of a woman at Lux nightclub in Belfast City Centre in the early hours of Sunday morning.
Police, paramedics and fire crews were called to the venue at about 02:20 GMT after reports that two women had fallen unconscious.
Police said that when they arrived one of the women, who was in her 20s, had died.
The other woman was taken to hospital where she is undergoing treatment.
In a statement, a spokesperson for Lux said: "Our thoughts and sympathies are with the parents and family during this very difficult time.
"Our focus is now to help the police with their investigation and our staff wellbeing."
The nightclub will be closed on Sunday evening.
Hospitality Ulster said it was "deeply saddened by the news".
"Nobody should ever go on a night out and not return home," its chief executive Colin Neil said in a statement.
Police have appealed for anyone with information to get in touch.