Five quantitative measures of diabetic control [HbA1c determinations, mean 24-h plasma glucose values, mean amplitude of glycemic excursions (MAGE), mean 24-h urinary loss of glucose, and daily exogenous insulin requirement] were compared in 20 pregnant women who were randomly assigned to either a high-carbohydrate, high-fiber diet (HCF) that was low in fat or to a control diet commonly prescribed for pregnancy. Eleven women followed the HCF diet and nine subjects, the control diet, from baseline entry into the study until delivery. Dietary compliance was excellent, with 78% of the women in each group rated good or acceptable. HbA1c values were similar in both groups at baseline (HCF: 11.0 ± 0.5% versus control: 10.2 ± 0.6%), with no different predelivery values (8.6 ± 0.4%). Mean 24-h plasma glucose levels improved in patients on both diets, with lower values noted in the HCF group at predelivery. MAGE values and standard deviations did not differ significantly in the two groups. Glycosuria decreased markedly in both dietary groups, but differences between groups were not significant. Improved control of diabetes on the HCF diet was achieved with significantly lower increments in insulin dose during gestation (HCF baseline: 32 ± 8 U/24 h to 66 ± 10 U/24 h versus control baseline: 27 ± 9 U/24 h to 108 ± 12 U/24 h, P < 0.03). Outcome of pregnancy did not differ in the two groups of patients, but women on the HCF diet gained less weight than those on the control diet (26 ± 3 lb versus 35 ± 5 lb, P < 0.05). Mean newborn gestational age was similar in the two groups (HCF: 37.2 ± 0.7 wk versus control: 36.5 ± 0.7 wk). Mean birth weight in infants of HCF mothers ± was 3809 ± 248 g versus 3313 ± 278 g in infants of control mothers (P < 0.05). We conclude that although marked improvement of diabetic control occurred on both regimens, patients on the HCF diet achieved better control of diabetes with significantly lower increments in exogenous insulin.