The objective was to test a method of assessing sun exposure for use in stratifying adolescent girls according to potential for vitamin D3 synthesis in the skin. Thirty-five girls wore polysulphone ultraviolet (UV) dosimeters and kept diaries of their activities on August 17, 2000. Minutes spent outdoors during each hour of the day were tabulated and adjusted for the fractional strength of ultraviolet B (UVB) radiation relative to peak hour. Total adjusted minutes outdoors were compared with UV dosimeter readings. A series of ampules containing 7-dehydrocholesterol were exposed on a flat plane to further investigate the potential for skin synthesis of previtamin D3 with a given sun exposure. The correlation between measured UVB exposure and self-reported minutes outdoors adjusted for the time of day was r = 0.64. In vitro previtamin D3 synthesis at midday was reduced by 50% on a cloudy day compared with a sunny day.