In order to examine the relevance of high sun protection factors (SPF) to medically important sunburn protection under natural exposure conditions, an all-day testing procedure using twenty-four sun-sensitive subjects was designed. Three small test areas above each scapula were used to compare sunscreen lotions with SPFs of 17 and 6 to a vehicle-treated site. In addition, all of the sun-exposed glabrous skin areas were treated with the highest SPF lotion. One side of each subject's torso and face was also retreated during the exposure period. The subjects were exposed to approximately fifteen times the minimal erythemal dose of ultraviolet B (UVB) during the test, and all received essentially complete protection with the SPF 17 lotion. No difference was seen between one and two applications. On the small test sites, both the SPF 17 and SPF 6 lotions provided protection from sunburn; however, the SPF 17 lotion provided significantly better protection than the SPF 6 lotion. Importantly, tanning was produced without associated or preceding redness, and these tanned areas persisted for 2 weeks or more. It was concluded that under natural exposure conditions when the integrity of the sunscreen film has not been compromised, a single application of the highest SPF sunscreen can protect sun-sensitive subjects from sunburn during a day-long sun exposure and that this protection is better than that afforded by a sunscreen with a substantially lower SPF.