Photosensitive patients often comment that sunscreen products seem of little benefit. We used fluorescence spectroscopy to assess quantitatively their sunscreen application technique. A dose-response relationship for sunscreen skin surface thickness and fluorescence intensity was determined for an intrinsically fluorescent sunscreen, Neutrogena sun protection factor (SPF) 15. Ten women with long-standing photosensitivity conditions were asked to apply this sunscreen in the manner they would normally on a bright sunny day. Fluorescence measurements were taken from all unclothed body areas, comprising 17 sites of the head, neck, upper and lower limbs. Geometric regression analysis of the dose-response data showed a high level of correlation (r = 0.99) between sunscreen thickness and fluorescence intensity, allowing fluorescence measurements to be converted to an equivalent sunscreen thickness. The overall median sunscreen thickness was 0.5 mg/cm2, with median thicknesses of individual sites ranging from 0 to 1.2 mg/cm2. The most frequently missed sites were the posterior neck, lateral neck, temples and ears, all of which had median thicknesses of 0 mg/cm2. Hence, photosensitive patients fail to apply sunscreen in some prominently exposed sites, and use average thicknesses far less than the manufacturers' recommendation (2 mg/cm2). The level of protection is much lower than anticipated from the stated SPF of the product.