A two-year-old Bolton boy died after suffering a cardiac arrest yesterday.
Officers launched a murder investigation after the boy was taken to Royal Bolton Hospital where he later died.
A 22-year-old man has been arrested on suspicion of murder and remains in police custody.
Detective Superintendent Jon Chadwick said: “I would like to extend my sympathies to this young boy’s family and friends at what is clearly a very difficult and emotional time for everyone.
“The investigation is in the early stages and a dedicated team of detectives will be working to establish exactly what happened.”
A Home Office post-mortem examination will take place at the Manchester Children’s Hospital to ascertain the cause of his death.