Abstract The aim of this paper has been to show the importance of the estimation of vitamin C level in the blood for the exact diagnosis of a real vitamin C deficiency. The concentration of vitamin C in the blood of 322 women (pregnant, post partum, nonpregnant) was measured. The average concentration was found to be 1.01 mg. per cent. In pregnant women (207 cases) the average blood concentration of vitamin C was 1.09 mg. per cent; in post-partum women (62 cases), the average concentration was 0.79 mg. per cent; in nonpregnant women (47 cases), the average concentration was 0.98 mg. per cent. In severe cases of hyperemesis gravidarum, the blood concentration of vitamin C was consistently low and averaged 0.7 mg. per cent. The low concentration is believed to be of secondary origin. Administration of vitamin C is therefore not causal therapy. The concentration of vitamin C in the blood of pregnant women was found to be 1.13 mg. per cent when the teeth were in good condition (17 cases); 1.00 mg. per cent when the teeth were in medium condition (27 cases); and 1.01 mg. per cent when the teeth were in bad condition (36 cases). The concentration of vitamin C in the blood of pregnant women suffering from gingivitis (43 cases) was normal, rating 1.18 mg per cent. The blood concentration of vitamin C when gingivitis was complicated by caries (7 cases) was slightly below normal, rating 0.9 mg. per cent. The concentration of calcium and phosphorus in the blood of pregnant women was found to be independent of the condition of the teeth. The concentration of vitamin C in human milk within the first nine days after confinement was measured (62 cases and 147 tests). The average concentration was found to be 4.62 mg. per cent. A continuous increase in the quantity of vitamin C in the milk beginning from the first day after birth was observed. The quantity of vitamin C excreted in the milk per day was calculated to be 2.4 mg. on the second day, and 27 mg. on the eighth day post partum. The vitamin C concentrations of milk and blood were not found to be definitely correlated. The age of the mother, the number of children previously borne, and the weight of the newborn infant do not influence the concentration of vitamin C in the milk and in the blood. The blood concentration of vitamin C after confinement was below normal (less than 0.75 mg. per cent) in 50 per cent of the patients examined. In certain patients, values as low as 0.32 mg. per cent were encountered. A survey of the concentration of vitamin C in the blood during different months of the year revealed the existence of a markedly higher average during the citrus season.