Introduction: Inadequate intake of breast milk in the first days of life leads to weight loss in neonates. Jaundice is also more prominent in these infants. In this cross-sectional study, we tried to evaluate the extent of weight loss in term breastfed infants who were hospitalized for idiopathic hyperbilirubinemia.Materials and Methods: This prospective cross sectional study involved 1072 infants>48 hours old, who were admitted to Ghaem Hospital of Mashhad-Iran, with idiopathic hyperbilirubinemia, between may 2007 - 2013. Infants were divided into two groups based on the amount of weight loss. Case groups (>7% weight loss) and control (weight loss£7%). The profile of maternal and NEONATAL risk factors were compared between them.Results: Average birth weight, gestational age, mode of delivery, maternal age, duration of mother's stay at hospital did not show any significant difference between two groups (P>0.05). Age at presentation, weight on admission, extent of weight loss, percentage of daily weight loss, duration of NEONATAL hospitalization, age at onset of hyperbilirubinemia, serum bilirubin and hematocrit levels were significantly different between two groups (p<0.05).Conclusion: According to this study's findings, about one thirds of neonates presenting with idiopathic hyperbilirubinemia, had severe weight loss and hyperbilirubinemia was also more severe in this group. The average weight loss in the neonates with severe hyperbilirubinemia (>20 mg/dl) was three times that of moderate hyperbilirubinemia (<20 mg/dl).