1. It is generally believed, though difficult to prove, that diet plays a role in the risk of various diseases. Components of difficulties include several issues such as dietary assessment method, regression dilution bias, multicolinearity and interaction among nutrients. 2. The present study focuses on colinearity and interaction between sodium and calcium, which should be cautiously examined in nutritional epidemiological studies in relation to blood pressure and bone mineral density. 3. The World Health Organization's International Cooperative Cardiovascular Diseases and Alimentary Comparison study showed significant multicolinearity among urinary sodium, calcium and urea nitrogen as well as urinary calcium and magnesium. Urinary sodium and calcium had significant correlation (r = 0.438, P < 0.05, n = 48) by cross-centre analysis. 4. Interaction between sodium and calcium on bone mineral density is studied using the data set from bone mineral density screening for 1658 females, aged 20-40 years, in Yokohama, Japan. Among those who have lower calcium intake (< 600 mg/day), higher calcium intake (%) from small fish, which is likely to be associated with a high salt diet, related to significantly lower mineral bone density. 5. Interaction between sodium and calcium on bone mineral density among young Japanese females is suggested. Moderate sodium restriction is needed for prevention of not only cardiovascular diseases but also osteoporosis.