OBJECTIVE : The primary purpose of this study was to develop an operational definition of the oral condition of ankyloglossia (also called tongue-tie) that occurs in newborns (i.e., age birth-6 months) and that could consistently be used in research studies. METHODS : This 4-round Delphi survey developed the consensus New York University-Tongue-Tie Case Definition (NYU-TTCD) by using a panel of ankyloglossia treatment experts. RESULTS : This tongue-tie case definition (TTCD) was carefully created in a step-wise manner from the bottom up by expert panelists over 4 rounds of inquiry. As a functioning case definition, it offers the diagnostician 2 separate pathways to identifying a newborn as being tongue tied. One pathway requires but a single pathognomonic anatomic feature, and the other pathway requires a single functional deficit accompanied by at least 2 of 12 other diagnostic items (functional, anatomic, or behavioral). CONCLUSIONS : This Delphi survey, as administered to a panel of ankyloglossia treatment experts, produced the first consensus case definition of tongue-tie for newborns (i.e., age birth-6 months) for use in epidemiologic research studies ranging from descriptive prevalence studies to clinical trials. Next-step studies should establish the validity, reliability, and utility of this novel NYU-TTCD case definition for epidemiologic and clinical purposes.