Twenty-one women completed a longitudinal study of glycosylated haemoglobin in normal pregnancy. Glycosylated haemoglobin levels were measured using two independent techniques (ion-exchange column and colorimetric). Concurrent serial oral glucose tolerance tests (75-g glucose load) and erythrocyte indices were obtained. Changes in mean glycosylated haemoglobin were similar with both techniques with a nadir at 17 weeks, a peak at delivery (p less than 0.002 versus 17 weeks) and a fall post-partum. Glycosylated haemoglobin levels in abnormal pregnancies, e.g. diabetic, should be interpreted in the knowledge of these physiological changes.