Infants with transient neonatal diabetes mellitus are small for gestational age and fail to thrive postnatally unless insulin is administered. We have measured the concentrations of insulin-related growth factors in an infant girl with this condition to learn if deficiencies in one or more of these factors could be responsible for the impaired growth. Cord blood serum radioimmunoassayable insulin and somatomedin C/insulin-like growth factor I (SMC/IGF-I) were low, but insulin-like growth factor II (IGF-II) measured by a specific radioreceptor assay was normal. Insulin therapy begun on the fourth day of life resulted i n a prompt increase in weight and a delayed rise in SMC/IFG-I. No significant changes in IGF-II were observed. After 2.5 months, insulin treatment was discontinued. At that time, endogenous insulin secretion was documented by increased urinary C-peptide. Normal growth and SMC/IFG-I levels persisted. We conclude that growth failure in this condition may be related not only to a lack of insulin but also to SMC/IGF-I deficiency. A deficiency in IGF-II is not involved.