Macrosomic infants still suffer birth trauma in excess of the general population; thus, while debated, the medical and legal sequelae of macrosomia appear to be costly. The clinical role of maternal hyperglycemia below the threshold for the diagnosis of gestational diabetes (GDM) in the etiology of macrosomia remains an area of controversy. Based on the hypothesis that increasing glucose levels result in an increasing prevalence of macrosomia, we designed a study to observe the impact on birth weight and on cost of a treatment program for glucose-intolerant pregnant women in The Santa Barbara County Health Care Services (SBCHCS). In 1985, 18% of 4364 births (85% Mexican-American in origin) in the SBCHCS were > 90th percentile birth weight. In 1986, we began a program to treat all glucose-intolerant pregnant women who had a positive glucose challenge test (GCT > 140 mg/dL after a 50-g oral glucose load), even if they had a negative glucose tolerance test. All glucose-tolerant pregnant women were placed on a 40% carbohydrate, 1800 kcal diet and taught to monitor their blood glucose. Insulin was begun if the fasting blood glucose was > 90 mg/dL and/or the 1-hour post meal was > 120 mg/dL. After introduction of the screening/ treatment program, the prevalence of macrosomia in 1992 was 7% and the cesarean section rate had dropped from 30 to 20%. The cost to SBCHC to educate and treat the additional glucose-intolerant women was $233,650. Assuming that there would have been an additional 398 macrosomic infants with some requiring cesarean delivery and intensive care, total potential savings could be estimated at $833,870 per year. Thus, treatment of glucose-intolerant pregnant women was associated with a decrease in macrosomia and may be cost-effective.