A supplementary heater can be mounted in trucks or other transportation vehicles and used to heat the air in the cabin independent of the engine of the vehicle. This apparatus works with diesel fuel or gasoline. Combustion products burning in the pre-combustion chamber heat the fins of the engine. The air passing through the fins is heated and is transferred into the cabin. The malfunction of such an apparatus may be the cause of carbon monoxide (CO) poisoning or fire. In this study, we report two cases in which drivers died while asleep in the cabins of parked trucks. In the first case, a 43-year-old man died because of CO poisoning originating from a broken supplementary heater. In the second case, a 48-year-old man died owing to a fire that resulted from the burning of upholstery cloths in the truck cabin, and which was caused by a supplementary heater set at very high temperatures. In both cases, it was determined by a technical expert that the CO poisoning in the first case and the fire in the second case were caused by the supplementary heater in the trucks. It is emphasized in this article that the supplementary heater in the truck might be the cause of mortal CO poisoning as well as be responsible for fires in the cabins of the trucks. It aims to highlight that a detailed investigation of supplementary heaters at the death scene is required for such death cases in the trucks. © 2008 Elsevier Ltd and FFLM.