An Airbus A330-322 passenger plane, D-AERK, sustained substantial damage when it lost engine cowlings after takeoff from Dsseldorf International Airport (DUS). The airplane landed safely back at Dsseldorf and no one was injured. The A330 had departed Dsseldorf on a flight to Thessaloniki International Airport (SKG), Greece. It was climbing through FL105 when the crew heard a loud bang, followed by some unusual noises. The crew then checked the locking of the doors and hatches but found no problems. Since no errors were detected by the aircraft monitoring system (ECAM) the flight continued. Shortly afterwards a cabin crew member reported the loss of a large part of the engine cowling of the left engine. The crew then decided to cancel the flight and return to Dsseldorf. The crew did not declare an emergency. The aircraft landed without further problems in Dsseldorf. At the parking position fuel was seen leaking from the left wing because of hole in the underside of the wing. Further damage was caused to the belly fairing and underlying structure and hydraulics. CONCLUSIONS: The accident was due to the fact that the front two or all three of the cowling latches were not properly closed. Contributing to the accident, was that the warning color on the handles was worn. There was no evidence of a structural failure of the composite cladding.