Background Ultraviolet (UV)-B light increases vitamin D levels, but the dose response and the effect of skin pigmentation have not been well characterized. Objective We sought to define the relationship between UVB exposure and 25-hydroxyvitamin D (25-OH-D) concentrations as a function of skin pigmentation. Methods Seventy two participants with various skin tones had 90% of their skin exposed to UVB light (20-80 mJ/cm 2) 3 times a week for 4 weeks. Serum 25-OH-D was measured weekly. Results Eighty percent of the variation in treatment response was explained by UVB dose and skin tone. Therapeutically important changes in 25-OH-D were achieved with minimal tanning. Limitations Four weeks was not long enough to reach a steady state at the higher dose rates. Conclusions The response of 25-OH-D levels to UVB light is dependent on skin pigmentation and the amount of UVB given, and useful increases in vitamin D status can be achieved by defined UVB doses small enough to produce only minimal tanning.