Aerolineas Argentinas Flight 737 departed Resistencia Airport (RES), Argentina on a domestic flight to Buenos Aires-Aeroparque (AEP) with an en route stop at Monte Caseros (MCS). Some 20 minutes after takeoff, while at cruising level of 3450 m, an explosive decompression of the cabin occurred. It appeared that the aft left passenger door had separated. A flight attendant was blown out and the airplane sustained severe damage to both aft bulkheads and the forward left bulkhead. The pilot-in-command decided to return to Resistencia at reduced power and speed . The aircraft landed at Resistencia without further incident. The airplane in question, LV-HHB, was lost in a fatal accident on 14 April 1976 when the right hand wing suffered a structural failure in flight, killing all 34 on board. Probable Causes: The separation of the passenger door in flight which was accompanied by violent decompression of the cabin was caused by failure of the locking system due to the following circumstances: (1) The door securing mechanism had not been properly overhauled in the absence of the manufacturer’s instructions regarding periodic checks to ensure correct operation, having regard to the delicacy of the mechanism and the lack of experience with aircraft of the subject type; (2) The method checking the engagement of the middle and lower latches of the door was not fully reliable .