OBJECTIVE: We hypothesized that young adults would commonly have vitamin D deficiency and that a questionnaire could help identify subjects with the condition. DESIGN: Between January and May 2004, we administered a questionnaire to a convenience sample of young adults. We measured each participant’s serum level of 25-hydroxyvitamin D (25(OH)D) using a chemiluminescent assay and defined deficiency as serum 25(OH)D < 16 ng/ml. SETTING AND SUBJECTS: We recruited young adults living in Madison, Wisconsin without pre-existing conditions affecting vitamin D and/or Ca metabolism. RESULTS: One hundred and eighty-four adults (mean age 24 years, 53 % women, 90 % Caucasian) participated in the study. Nearly three in four adults (71 %) had 25(OH)D level <30 ng/ml and 26 % were vitamin D-deficient. In multivariate analysis, persons reporting a suntan (OR = 0.24, 95 % CI 0.09, 0.63, P = 0.004), tanning booth use (OR = 0.09, 95 % CI 0.02, 0.43, P = 0.002) and daily ingestion of two or more servings of milk (OR = 0.21, 95 % CI 0.09, 0.48, P < 0.001) were less likely to be deficient. These three questions provided a sensitivity and specificity of 79 % and 78 %, respectively, for the presence of deficiency. CONCLUSIONS: The questionnaire is moderately useful to identify young adults likely to be vitamin D-deficient. Additional revisions of the questionnaire may improve its ability to predict vitamin D deficiency.