To explore the relation between reduced fetal growth and impaired glucose tolerance in adult life, an oral glucose tolerance test (75 g glucose) was carried out on 218 men and women, now aged around 50 years, who had been measured in detail at birth. Measurements of plasma concentrations of glucose and insulin were made at 0, 30, and 120 min. Fasting plasma concentrations of proinsulin and 32-33 split proinsulin were also measured. People in the highest category of birthweight tended to have the lowest plasma concentrations of insulin as adults at both 0 and 120 min, though both these relations were weak. Plasma insulin concentrations in adult life were more strongly related to abdominal circumference at birth than to birthweight. After adjusting for sex and body mass index, mean insulin concentrations at 0 min fell from 50 pmol l-1 to 46 pmol l-1 (p = 0.04) and at 120 min from 235 pmol l-1 to 144 pmol l-1 (p = 0.003) between people whose abdominal circumference at birth had been less than 11.5 in and those who abdominal circumference had been greater than 13 in. Plasma glucose concentrations at 120 min also fell with increasing abdominal circumference at birth. Because abdominal circumference at birth is an indicator of the growth of the liver in fetal life, one interpretation of these findings is that the sensitivity of the liver to insulin is permanently reduced if the intrauterine development of this organ is impaired.