We studied the relationship between the metabolic control of pregnant diabetics and fetal lung maturity. In 31 diabetic and 20 normal pregnancies we analysed phospholipids in amniotic fluid and glycaemic control parameters. There were no differences in amniotic fluid lecithin/sphingomyelin (L/S) and phosphatidylinositol/sphingomyelin (PI/S) ratios between pregnant diabetics with strict metabolic control and the control group. Pregnant diabetics with poor metabolic control had significantly different L/S and PI/S ratios than the normal pregnant women. Six women in this group of poorly controlled diabetics had mature surfactant in amniotic fluid (L/S > or = 2.7 and presence of phosphatidylglycerol) at 32-34 weeks of amenorrhea; the difference was significant (p < 0.05) with respect to the control group. These six women had recurrent hypoglycaemic episodes (6.4 +/- 1.3 episodes/week) at 14-27 weeks of amenorrhea. Our findings suggest that hypoglycaemic stress on the fetus could disturb fetal synthesis of pulmonary phospholipids.