Background/Purpose: Vitamin D3 plays important roles in the absorption of calcium and phosphorus from the gastrointestinal tract and in the treatment of rickets; in addition, it facilitates the deposition of minerals in bones, thus minimizing the possibility of developing osteomalacia. Sunlight naturally induces vitamin D3 photosynthesis. Such a process is affected by a number of factors such as age, geographical location, skin color, sunscreen application and clothing. It is intended in the present investigation to study in vitro the effect of clothing on the solar photoproduction of vitamin D3. Methods: Fifteen different fabric samples were tested for their effect on the efficiency of the in vitro solar conversion of 7-dehydrocholesterol (7-DHC) to vitamin D3. 7-DHC was dissolved in methanol to give a concentration of 2.6×10−4 M. Solutions were exposed to sunlight in quartz containers for predetermined periods either uncovered or covered with the fabric sample under test. Changes in the concentrations of 7-DHC and the photoproducts were monitored by HPLC. Fabrics were graded as the number of threads per square inch (in2), and their sunlight attenuation was determined. Results: 7-DHC is transformed to previtamin D3 upon exposure to sunlight, and the amount generated exhibited an almost linear relationship. When fabric-covered samples of 7-DHC were irradiated, photoproducts were also detected and their concentrations depended on the degree of sunlight attenuation imposed by the fabric. Generally, the higher the number of threads per in2 the more the light attenuation produced. Conclusion: Clothing plays an important role in attenuating sunlight, thus leading to diminished vitamin D3 production to an extent that would require dietary compensation.