In 25 normally non-pregnant women, 543 normally pregnant women and 75 pregnant women with diabetes mellitus or gestational diabetes mellitus, the relationship between the serum concentration of 1,5-anhydro-D-glucitol (1-deoxy-glucose) and carbohydrate metabolism was studied. The concentration of 1,5-anhydro-D-glucitol was estimated by means of gas-liquid chromatography. In normally non-pregnant women the concentration was found to be 18.6 +/- 5.2 mg/l (mean +/- SD). During the normal pregnancy, from 9 weeks of gestation, a steadily decreasing concentration was observed as the pregnancy progressed and the lowest value (10.2 +/- 4.6 mg/l) was found in the third trimester. After 5 days of puerperium the concentrations were found to be 10.8 +/- 3.7 mg/l. On the 30th day postpartum, the level was within the range for non-pregnant subjects. The values in pregnant women with diabetes mellitus and gestational diabetes mellitus were mostly below 10 mg/l throughout the entire pregnant period. The 1,5-anhydro-D-glucitol concentration was not affected by meals or oral glucose loading. A concentration below 10 mg/l was found in 36% of the normally pregnant women, where oral glucose tolerance tests and measurement of glycohemoglobin were shown to be within the normal range. The present study suggests that a change of 1,5-anhydro-D-glucitol level during pregnancy may reflect a mild alteration of carbohydrate metabolism that goes undetected by all the other diabetic indicators.