Two nonmethodological explanations for a left-sided excess of melanoma can be speculated. Traveling in a motor vehicle is probably the only frequent human activity that results in side-specific UV exposure depending on the individual position in the car. Swiss drivers sit on the left side of the car and, until the recent availability of air conditioning, their left arm was more likely to be sun exposed through an open window, particularly in summertime. The largest left-sided excess observed for the upper limbs (an L/R ratio of 1.27, 95% CI: 1.05-1.54,Table) and the greater L/R ratio for men (an L/R ratio of 1.38, P = .02, data not shown) than women (an L/R ratio of 1.18, P = .22, data not shown) at this site supports this assumption and the known greater propensity for men to drive. Reports of a left-sided excess of facial photodamage lesions commensurate with time spent driving in the United States4 and the commoner occurrence of solar keratoses on the right upper limb among Australian men,3 where drivers sit on the right side of vehicles, concurred with our findings. This hypothesis, however, only partly explains our results, since it cannot account for the left-sided excess of melanomas observed at other body sites.