Nutrition practice guidelines were developed for gestational diabetes mellitus by registered dietitians from the Diabetes Care and Education and the Women's Health and Reproductive Nutrition dietetic practice groups. To validate the guidelines, a clinical trial was designed with clinic sites randomly assigned to either nutrition practice guidelines care (12 sites) or usual nutrition care (13 sites), with diabetes, obstetric, and other clinic types represented in both groups. Volunteer dietitians served as study coordinators and recruited women diagnosed with gestational diabetes mellitus. The nutrition practice guidelines define medical nutrition therapy (MNT) for gestational diabetes and emphasize three areas-definition of MNT clinical goals with indexes to modify or advance MNT and criteria to start insulin; use of self-monitoring tools; and provision of three nutrition visits. Usual care sites provided prenatal nutrition care according to usual practice. The effect of nutrition care (sites following the nutrition care guidelines) and type of clinic site on changes in glycated hemoglobin and infant birth weight, adjusted for other covariates, were evaluated using linear regression. Differences in insulin use and other infant outcomes between treatment groups were evaluated using logistic regression. Generalized estimating equations were used to accommodate nonindependence within randomized clusters of patients within clinic sites. Data from 215 women indicated less insulin use at diabetes clinic sites in the nutrition practice guidelines groups and improved glycated hemoglobin control during the treatment period in diabetes clinics compared with obstetric or other clinics. A higher proportion of women in the usual care group had glycated hemoglobin levels that exceeded 6% at follow-up compared with women in the nutrition practice guidelines group (13.6% vs 8.1%), although not statistically significant (P=0.26). A significant clinic type and treatment group effect was found for birth weight. Nutrition practice guidelines for gestational diabetes mellitus reflected nutrition care already being provided by registered dietitians in diabetes clinics prior to this study because outcomes at these clinics were not impacted. Use of the guidelines by dietitians at obstetric and other clinics tended to improve outcomes at these sites.