Gross epidemiological data indicate there are no significant differences in rates of aging osteopenia among countries with substantially different amounts of Ca in their national food supplies. This-observation, plus the fact that Ca administration fails to reverse osteoporotic bone loss, has led some investigators to conclude that Ca nutrition is an insignificant factor in the etiology of osteoporosis. However, it has become apparent that a Ca intake that may be adequate for adults consuming a low protein, low P, neural, or alkaline cereal-based diet is not necessarily adequate for subjects consuming a high protein, high P, acidic mixed Western diet. Ca administration inhibits postmenopausal osteopenia and there is epidemiological evidence that a liberal Ca intake reduces bone loss in middle adulthood. Ca intakes in the United States and Canada appear generally satisfactory among children and young adults, but low intakes by many individuals of middle age is a cause for concern, especially among women. Although the Ca:P ratio for the average diet consumed in these countries (about 1:1.6) appears to be satisfactory, a low intake of dairy foods, coupled with a high intake of other foods rich in natural and added phosphorus, may raise the ratio above 1:2, a value beyond which animal studies indicate that there is a risk of increased bone loss.