Previous studies have shown that people often apply less sunscreen than the recommended amount of 2 mg/cm2. Our purpose in this study was to determine objectively how photoprotection varies with application thickness. The protection provided by differing quantities of sunscreens containing varying amounts of titanium dioxide was measured in vitro using excised human epidermis as the substrate. It was found that application thickness had a significant effect on the sun protection factor (SPF), with most users probably achieving a mean SPF of between 20–50% of that expected from the product label. Underprotection due to inadequate application, coupled with overexposure to the sun, might partially explain why sunscreen use has been reported to be a risk factor in melanoma.