Objectives: The aims of this study were to assess the morphological characteristics and to establish ultrasonographic standards of normal neonatal insula size using transfontanellar ultrasonography, and to evaluate the clinical value of this technique. Materials and Methods: The authors performed transfontanellar ultrasonography in 481 single-birth cases at 28-43 weeks' gestation. Ultrasonographic examinations were performed in the parasagittal plane at the level of the insula through the anterior fontanelle, measuring area, and perimeter of the insula. Regression analyses were used to evaluate the relationship between insula size and gestational age (GA), and 60 cases were randomly selected for assessment of intra-observer and inter-observer reliability of ultrasonographic measurements. The authors obtained standard values of normal insula size and used them to assess and follow-up 40 cases with suspected insular malformations. Additionally, 30 late-onset neonates who were determined as small for gestational age (SGA) and 45 normally growing neonates were examined and tested using the Neonatal Behavioral Assessment Scale (NBAS). Results: The neonatal insula appeared as an inverted triangle, with the insular gyri extending radially in an anterior-inferior to posterior-superior direction. The area and perimeter of the normal neonatal insula significantly increased with GA (p < 0.01 for both), and these measurements were highly reliable. This assessment of cases with suspected insular malformation showed that five out of 40 cases presented abnormalities. Late-onset SGA neonates presented a significantly smaller area and perimeter in the insula compared to controls. In addition, the measured values of the insula significantly correlated with NBAS scores. Conclusion: Evaluation of the neonatal insula using transfontanellar ultrasonography can be performed and is clinically useful.