Low intake of nutrients is associated with poor health outcomes. We examined the contribution of dietary supplementation to meeting recommended dietary intakes of calcium, magnesium, potassium, and vitamin C in participants of the Multi-Ethnic Study of Atherosclerosis, a cohort of white, African-American, Hispanic, and Chinese-American participants ages 45 to 84 years. We also assessed the prevalence of intakes above Tolerable Upper Intake Levels (ULs). At the baseline exam in 2000-2001, 2,938 men and 3,299 women completed food frequency questionnaires and provided information about dietary supplementation. We used relative risk regression to estimate the probability of meeting Recommended Dietary Allowances (RDAs) or Adequate Intakes (AIs) in supplement users vs nonusers and Fisher's exact tests to compare the proportion of those exceeding ULs between the two groups. RDAs, AIs, and ULs were defined by the National Academy of Sciences Food and Nutrition Board's Dietary Reference Intakes (DRIs). After adjustment for age and education, the relative risk of meeting RDAs or AIs in supplement-users vs nonusers ranged from 1.9 (1.6, 2.3) in white men to 5.7 (4.1, 8.0) in African-American women for calcium, from 2.5 (1.9, 3.3) in Hispanic men to 5.2 (2.4, 11.2) in Chinese men for magnesium, and from 1.4 (1.3, 1.5) in African-American women to 2.0 (1.7, 2.2) in Chinese men for vitamin C. The relative risks for meeting RDAs for calcium differed significantly by ethnicity (P<0.001) and sex (P<0.001), and by ethnicity for magnesium (P=0.01). The relative risk for each sex/ethnicity strata was close to 1 and did not reach statistical significance at alpha=.05 for potassium. For calcium, 15% of high-dose supplement users exceeded the UL compared with only 2.1% of nonusers. For vitamin C, the percentages were 6.6% and 0%, and for magnesium, 35.3% and 0% (P<0.001 for all). Although supplement use is associated with meeting DRI guidelines for calcium, vitamin C and magnesium, many adults are not meeting the DRI guidelines even with the help of dietary supplements, and the effect of supplementation can vary according to ethnicity and sex. However, supplementation was not significantly associated with meeting DRIs for potassium. Also, high-dose supplement use is associated with intakes above ULs for calcium, magnesium, and vitamin C.