Multivariate methods were used to identify risk factors for macrosomia (birth weight > 4000 g) among 741 singleton births to Native Canadian women from Sioux Lookout Zone, Ontario, Canada, in 1990-1993. The average birth weight was 3691 +/- 577 g, and 29.2% of infants weighed more than 4000 g at birth. Macrosomic infants were born at later gestational ages and were more likely to be male. Women delivering macrosomic infants were taller, entered pregnancy with higher body mass indexes (BMI) and gained more weight during pregnancy, but were less likely to smoke cigarettes. They were more likely to have previously delivered a macrosomic infant and to have had gestational diabetes mellitus (GDM). Risk of macrosomia was associated with maternal glycemic status; women with pre-existing diabetes were at greatest risk, followed by those with GDM A2 (fasting glucose > or = 6 mmol/l). Women with GDM A1 (fasting glucose < 6 mmol/l) were not at increased risk for delivering a macrosomic infant, but glucose-tolerant women with high glucose concentrations 1 h after the 50 g challenge were at somewhat increased risk. Maternal glycemic status and maternal nutritional status before and during pregnancy are important determinants of macrosomia in this native population.