We report a female small for date neonate, who developed transient diabetes mellitus (TDN) five days after birth and required insulin therapy for five weeks. At the onset of the disease, plasma insulin concentration was extremely low. At four weeks of age, after insulin withdrawal, the patient was still hyperglycemic, and basal insulin values assayed over a period of 24 h were mostly inadequate. Glucagon secretion was not suppressed. Growth hormone levels were lower than those of three small for date infants of the same age. At three months of age, the patient was still intolerant to an oral glucose load, insulin secretion remained inadequate while glucagon paradoxically increased 30 min after glucose challenge. The oral glucose tolerance values were in the normal range at six months of age. We conclude that TDN is caused by a transitory defect of insulin secretion, which would also explain the glucagon response as a consequence of insulin deficiency. We found no evidence associating the insulin antagonists observed in our study with the pathogenesis of this illness.