As insulin is a major fetal growth hormone, we have related the mother's nutrient intakes (assessed by a food frequency questionnaire) and other influences associated with fetal growth to the baby's concentrations of insulin and its propeptides in umbilical cord plasma. Among 391 term babies studied, those whose mothers had high energy intakes in early pregnancy and low protein intakes in late pregnancy had lower cord plasma concentrations of 32-33 split proinsulin, insulin, and C-peptide. Concentrations of split proinsulin fell by 0.66 (95% Cl 0.29 to 1.03, p = 0.0006) log pmol l-1 for each log kcal increase in the mother's energy intake in early pregnancy and by 0.005 (95% Cl 0.000 to 0.010, p = 0.04) log pmol l-1 for each g decrease in protein intake in late pregnancy. Insulin and propeptide concentrations were however unrelated to the mother's height and body mass index, and to smoking during pregnancy. These observations parallel recent studies relating the same pattern of dietary intakes to impaired fetal and placental growth. Although dietary intakes assessed by food frequency questionnaires allow only cautious conclusions, our findings could have implications for the offspring's risk of Type 2 diabetes mellitus in adult life.