Vitamin D may be associated with reduced risks of several types of cancer, including colon, prostate, and breast. We examined the relationship between vitamin D -related questions administered in a telephone interview and serum 25-hydroxyvitamin D [25(OH)D]. Three hundred and eight eligible women were randomly selected from controls in a breast cancer casecontrol study. Questions pertaining to sun exposure and dietary sources of vitamin D over the previous 4 weeks were asked in both summer and winter. We assessed the association between questionnairederived items and 25(OH)D using multiple linear regression. There were 217 participating women, 203 in summer and 213 in winter. Models were adjusted for age, body mass index, and skin color. Number of days when more than 0.5 hour was spent outdoors per