A 30-year-old man has been arrested on suspicion of murdering a three-year-old girl, police said.
An ambulance was called to an address in Beck Road, Shiregreen, Sheffield, on Friday evening and a girl was taken to the city's Children's Hospital.
She died at 12.20am at the hospital on Saturday, despite the efforts of medics.
A South Yorkshire Police spokesman said: "A 30-year-old man from Beck Road, Shiregreen, has been arrested on suspicion of murder and enquiries are continuing."
Detectives have appealed for anyone with information to call South Yorkshire Police on 101.