Fetal hyperinsulinism in infants of diabetic mothers (IDMs) produces increased fetal growth leading to macrosomia, which may or may not be proportionate. Disproportionate macrosomia refers to excessive weight characterized by a high weight/length ratio. We tested the hypotheses that (1) macrosomia in IDMs would be characterized by a high ponderal index (defined as weight/length ratio) and (2) infants with macrosomia who have a high ponderal index would have increased neonatal morbidity--specifically, hyperbilirubinemia, hypoglycemia, polycythemia, and acidosis. We studied 170 IDMs and 510 non-IDMs matched 1:3 for gestational age, race, and year of delivery. Forty-five percent of IDMs had macrosomia compared with 8% of control infants (p = 0.001), and 19% of IDMs had disproportionate macrosomia compared with 1% of control infants (p = 0.001). The rates of hyperbilirubinemia (p = 0.02), hypoglycemia (p = 0.01), and acidosis (p = 0.01) were greatest in infants with disproportionate macrosomia and least in nonmacrosomic infants. The incidence of polycythemia was not significantly different between the groups. We suggest that disproportionate macrosomia in the IDM is associated with an increased likelihood of neonatal complications.