Cutaneous vitamin D3 synthesis and release into the circulation is promoted by skin exposure to ultraviolet B radiation (UVB, spectrum 290 to 320 nm). To determine the relation between UVB energy level and cutaneous vitamin D synthetic response, we delivered graded increases of UVB suberythemic radiant energy (3 to 27 millijoules/cm2 [mJ/cm2]) to 32 untanned young white subjects with skin type III (Fitzpatrick-Pathak classification). Serum vitamin D3 was determined 1 hour before (basal value) and 24 hours after a single whole body exposure to UVB in a phototherapy unit. The basal vitamin D3 concentration was similar in all individuals (mean +/- SEM for whole group, 1.6 +/- 0.2 ng/ml). UVB irradiances were followed by proportional rises in serum vitamin D3 (at 27 mJ/cm2, 14.3 +/- 3.7 ng/ml), and the overall correlation between UVB radiation and consequent serum vitamin D3 response (r = 0.81; p less than 0.02) was best described by an exponential function. The minimal UVB radiation level that produced a significant increase in serum vitamin D3 was 18 mJ/cm2, a value similar to the lowest solar broadband UVB irradiance that generates previtamin D3 in vitro from the precursor 7-dehydrocholesterol (20 mJ/cm2). Because in the northern United States winter UVB irradiance does not generally reach this threshold level, we conclude that individuals living at extreme northern (or southern) latitudes may have higher dependence on body stores and dietary supply to meet their vitamin D requirements during winter.