Sunscreens block the cutaneous absorption of UV-B radiation and prevent sunburning, premature aging, and cancer of the skin. Inasmuch as UV-B radiation is also responsible for the photosynthesis of vitamin D3, we investigated the effect of sunscreens on the cutaneous formation of vitamin D3 in vivo and in vitro. Eight normal subjects, four of whom had been protected with the sunscreen para-aminobenzoic acid (sun protection factor 8), were exposed to one minimal erythema dose of UV radiation. The mean serum vitamin D3 concentration increased from 1.5 +/- 1.0 (+/- SEM) to 25.6 +/- 6.7 ng/mL in unprotected subjects, whereas it was 5.6 +/- 3.0 and 4.4 +/- 2.4 ng/mL at these times in the subjects who were protected with para-aminobenzoic acid. Para-aminobenzoic acid also prevented the photoisomerization of 7-dehydrocholesterol to previtamin D3 in human skin slices in vitro. These results indicate that the sunscreen interferred with the cutaneous production of vitamin D3.