In this prospective study 246 women with gestational diabetes were followed up to determine the characteristics of metabolic control associated with large-for-gestational-age infants. Memory-based reflectance meters were used for self-monitoring blood glucose. Ambulatory glucose profiles were produced to characterize glycemic control levels throughout pregnancy. With these novel approaches to the collection and representation of glucose data, the severity of glucose intolerance (hyperglycemia) was found to be associated with both maternal and neonatal morbidity in terms of infant size and cesarean section rate. By use of hierarchical cluster analysis to identify three groups on the basis of control levels (low less than 87 mg/dl, mid 87 to 105 mg/dl, high greater than 105 mg/dl) we were able to show a positive outcome in the low group with reduced rates of large-for-gestational-age (2%) and macrosomatic (0%) infants. Furthermore, we showed that as mean blood glucose levels and instability in glycemic control increased from group to group, incidence of large-for-gestational-age and macrosomatic infants increased. Whereas obesity increased the relative risk of adverse neonatal outcome, type of treatment (insulin versus diet) did not appear to be significant. Appropriately monitored toward stability within a narrow range to achieve tight metabolic control, ambulatory glycemia in pregnancy is associated with a decreased risk of maternal and fetal complications.