Serial human placental lactogen (H.P.L.) determinations were carried out in 98 diabetic women during the third trimester of pregnancy. H.P.L. levels were consistently higher than those in normal pregnant women. When patients were classified according to the severity of their diabetes (White classification), no significant differences in H.P.L. were detected between groups. Changes in blood-sugar during the day did not affect H.P.L. readings, and insulin requirements during pregnancy could not be related to H.P.L. levels. 10 pregnancies ended in fetal death, and in 4 of them H.P.L. levels were persistently below 4 mug. per ml. Of these 4 infants only 1, whose intrauterine death remains unexplained, could have been saved, 2 having fatal malformations and another infant having died during an episode of maternal ketoacidosis. Among the 6 unsuccessful pregnancies in which H.P.L. levels were greater than 4 mug. per ml., congenital malformations accounted for 5 losses and hyaline-membrane disease for a single neonatal death. These results indicate that H.P.L. determinations probably have a very limited role in the successful management of diabetic pregnancy.