Sunscreen use is often proposed for sun protection because of their ability to block UV-induced sunburns (the sun protection factor--SPF). Among suntan seekers, however, risk of cutaneous melanoma may be increased because of extended sun exposure duration. We made a systematic review of the evidence linking sunscreen use to sun exposure duration. Five observational studies found that when sun exposure was associated with willingness to get a tan or to stay longer in the sun (i.e., intentional sun exposure), sunscreen use was associated with duration of sun exposure 13-39% longer. Paradoxically, sunburns tend to be more frequent among sunscreen users, probably because of greater natural sun sensitivity. When sun exposure was not intentional, sunscreen use did not increase time spent in the sun. Two European double-blind randomized trials conducted among young sun seekers found daily sun exposure duration, especially sunbathing, 19-25% longer with use of SPF 30 than with use of SPF 10 sunscreens. One randomized trial in a holiday resort in France found a 3-13% increase in sun exposure duration with use of SPF 12 versus SPF 40 sunscreen. But, the SPF 12 groups used 3.6-4.2 more sunscreen than the SPF 40 group, and thus the actual SPF in the SPF 12 group was higher than in the SPF 40 groups. In conclusion, sunscreen use leads to longer duration of sun exposure when sun exposure is intentional, but not when sun exposure is non intentional.