The purpose of this study was to examine the effects of summer sun exposure on serum 25-hydroxyvitamin D [25(OH)D], calcium absorption fraction, and urinary calcium excretion. Subjects were 30 healthy men who had just completed a summer season of extended outdoor activity (e.g. landscaping, construction work, farming, or recreation). Twenty-six subjects completed both visits: after summer sun exposure and again approximately 175 d later, after winter sun deprivation. We characterized each subject's sun exposure by locale, schedule, and usual attire. At both visits we measured serum 25(OH)D, fasting urinary calcium to creatinine ratio, and calcium absorption fraction. Median serum 25(OH)D decreased from 122 nmol/liter in late summer to 74 nmol/liter in late winter. The median seasonal difference of 49 nmol/liter (interquartile range, 29-67) was highly significant (P < 0.0001). However, we found only a trivial, nonsignificant seasonal difference in calcium absorption fraction and no change in fasting urinary calcium to creatinine ratio. Findings from earlier work indicate that our subjects' sun exposure was equivalent in 25(OH)D production to extended oral dosing with 70 micro g/d vitamin D(3) (interquartile range, 41-96) or, equivalently, 2800 IU/d (interquartile range, 1640-3840). Despite this input, at the late winter visit, 25(OH)D was less than 50 nmol/liter in 3 subjects and less than 75 nmol/liter in 15 subjects.