Objective: Each year children die from heat stroke after being left unattended in motor vehicles. In 2003 the total was 42, up from a previous average of 29 for the past four years. Through August 8, 2004 there have been 20 child fatalities in vehicles due to hyperthermia. Heat stroke occurs when the body temperature reaches approximately 104 degrees F and body temperatures of 107 degrees or greater are often fatal. The problem is exacerbated with children because their body temperature rises at a rate 3 to 5 times faster than adults. Previous research showed the temperature rise in vehicles is significant when the ambient temperature is greater than 90oF. This study evaluates a full spectrum of temperatures from mild to hot ambient conditions. Methods: An observational study of temperature rise over time was measured in a dark sedan on 16 different days with ambient temperatures ranging from 72-96oF. On two of these days additional measurements were made with the windows opened 1.5 inches. Results: Regardless of the ambient temperature the rate of temperature rise inside the vehicle was consistent through the entire range of temperatures. It was observed that the majority of temperature occurred during the first 30 minutes. The opening of the windows did not significantly decrease the temperature rise. Conclusions: Even in relatively mild ambient temperatures temperature rise in enclosed vehicles is significant and would put infants in danger of hyperthermia. Vehicles heat up rapidly in the sun with the majority of the temperature rise in the first 15 to 30 minutes. Leaving the windows opened slightly does not slow the heating process.