BACKGROUND: In epidemiologic studies, sunscreen use is associated with increased risk of cutaneous melanoma, basal cell skin cancer, and higher numbers of nevi. It has been proposed that sunscreens may encourage prolonged sun exposure because they delay sunburn occurrence. We examined whether, under habitual conditions of sunscreen use, the sun-protection factor (SPF) had an influence on sun-exposure duration. METHODS: Before the 1997 summer holidays, we randomly assigned 87 French and Swiss participants who were 18-24 years of age to receive an SPF 10 or an SPF 30 sunscreen. Neither medical personnel nor study participants were aware of their sunscreen assignment. Participants were asked to complete daily records of their sun exposure. To avoid influencing the recreational sun-exposure habits of the study participants, no recommendation was made about sun exposure or sun protection. Furthermore, participants were told that the trial end point was the number of pigmented skin lesions before and after the holidays. One subject was lost to follow-up. All statistical tests were two-sided. RESULTS: The SPF 10 (n = 44) and SPF 30 (n = 42) groups had equivalent mean holiday durations (19.4 days versus 20.2 days) and mean quantities of sunscreen used (72.3 g versus 71.6 g). The mean cumulative sun exposures for the two groups were 58.2 hours and 72.6 hours, respectively (P =.011). The mean daily durations of sunbathing were 2.6 and 3.1 hours, respectively (P =.0013), and, for outdoor activities, they were 3.6 and 3.8 hours, respectively (P =.62). There was no difference in sunburn experience between the two groups. CONCLUSIONS: Use of higher SPF sunscreen seems to increase the duration of recreational sun exposure of young white Europeans.