The monthly variation of Road Traffic Accidents (RTAs) in Riyadh city in the period 1989-1993 has been studied with reference to time of day, lighting conditions and prevalent weather conditions. Total RTA accidents were significantly more common, being directly correlated, with increased dry and wet bulb temperatures and significantly less common, being inversely correlated, with increased relative humidity and amount of precipitation of rain, snow, hail etc. However, RTAs recorded on rainy days only were significantly more common and directly correlated with precipitation (Note days of snow and hail are very rare in Riyadh city). Seasonal variation in RTAs was evident being maximal during the summer season particularly between 12 noon and 3 pm. This period is characterized by heavy traffic and intense sunlight. The role of hot weather prevalent in Saudi Arabia, where average temperatures of 34.4-34.7 degrees C with maximum of 40-42.7 degrees C are common in summer, have been suggested to be an important factor leading to increased stress and decreased performance of intellectual tasks which require considerable physical effort and motor skills. Increased heart rates, exacerbation of existing pathologic conditions such as heart disease and emphysema and loss of visual acuity have been reported. Consequently, prolonged exposure to heat must be considered as a hazard to the safety and health of drivers and a factor leading to an increased incidence of RTAs.