The spot and 24 hour urinary urea nitrogen/total nitrogen (UN/TN) ratio was investigated as an index of maternal protein nutrision, in both outpatients and patients under metabolic study This ratio was found to correlate well with maternal protein nutrition. It was not influenced by caloric intake. In all the phases of this study, there was an excellent correlation between the UN/TN ratio in the 24 hour and the single voided urine specimens. Thus, the test was found to be a simple, reliable, and clinically feasible means of assessing maternal protein nutritional adequacy. Preliminary observations showed a positive correlation between the UN/TN ratio and birth weight. The historical background of the UN/TN ratio as well as its implications for the future are discussed.