Care International staff have succeeded in restoring electricity to the Yarmuk general hospital in western Baghdad.
Engineers working for the humanitarian organisation have repaired the generator at the 1,200-bed teaching hospital, one of the largest in the city.
The hospital was hit by shells when US troops entered the Iraqi capital and has been looted in the subsequent collapse of law and order.
"The health situation is critical," said Margaret Hassan, director of Care International Iraq.
"The most urgent need in hospitals is to restore electricity. Without it, operations cannot be performed and refrigerated insulin and vaccines would spoil."