Comprehensive measures to evaluate the effectiveness of medical interventions in extremely preterm infants are lacking. Although length of stay is used as an indicator of overall health among preterm infants in clinical studies, it is confounded by nonmedical factors (e.g. parental readiness and availability of home nursing support).To develop the PREMature Infant Index (PREMIIWe developed the PREMII according to US Food and Drug Administration regulatory standards. Development included five stages: (1) literature review, (2) clinical expert interviews, (3) Delphi panel survey, (4) development of items/levels, and (5) cognitive interviews/usability testing. Scoring approaches were exploredKey factors reflective of functional status were identified by physicians and nurses during development of the PREMII, as were levels within each factor to assess functional status. The resulting PREMII evaluates eight infant health factors: respiratory support, oxygen administration, apnea, bradycardia, desaturation, thermoregulation, feeding, and weight gain, each scored with three to six gradations. Factor levels are standardized on a 0-100 scale; resultant scores are 0-100. No usability issues were identified. The online clinician survey identified optimal scoring methods to capture functional status at a given time point.Our findings support the content validity and usability of the PREMII as a multifunction outcome measure to assess functional status over time in extremely preterm infants. Psychometric validation is ongoing.