A female hospital worker has been arrested in connection with the suspicious deaths of four children.
Thames Valley Police arrested the unidentified member of staff at Wexham Park Hospital at Slough, Berkshire in England last night.
Police were called in after consultant paediatricians raised the alarm about the unusually high death rate on the children's medical ward.
A spokeswoman for the Heatherwood and Wexham Park Hospitals Trust said no exceptional features emerged in the routine review of the deaths and post-mortems proved inconclusive.
She added: "However, when all of the cases were reviewed together, the fact that there were four such deaths contrasted with the annual average of two or three.
"The trust has therefore taken the concerns very seriously and brought in Thames Valley Police."
A helpline is being set up for worried relatives.