To evaluate the heat burden in parked cars, a study was done to measure inside car temperatures in different situations. A large car and a small car were parked in direct sunlight and in shade. In direct sun, the highest temperatures recorded in the small and large cars were 78 C and 65 C, respectively. The highest temperature in shade for the small car was only 44 C. There was a tremendous buildup of heat inside the car parked in direct sun as opposed to shade (p less than 0.001), and the small car heated more quickly than the large car under similar conditions (P less tan 0.001). Leaving the windows partially or fully open is not protective. With partial ventilation the highest recorded temperature in the small car was 70 C. These findings suggest that the heat burden of poorly ventilated, parked cars exposed to direct sun can be enormous. The common practice of leaving infants, toddlers, elderly people, and pets in cars under these conditions could cause them tremendous heat stress. The potential hazards of this practice should be recognized and special precautions should be taken.