Objective: Infant colic (IC) is defined as recurrent and prolonged crying without an obvious cause or evidence of failure to thrive or illness. It is a common problem with a prevalence of 5%-25%. The unknown aetiology results in a wide variety in interventions and use of heterogeneous outcome measures across therapeutic trials. Our aim was to develop a core outcome set (COS) for IC to facilitate and improve evidence synthesis. Design and setting: Prospective study design; primary, secondary and tertiary care. Methods: The COS was developed using a modified Delphi technique. First, healthcare professionals (HCPs) and parents of infants with IC were asked to list up to five outcomes they considered relevant in the treatment of IC. Outcomes mentioned by >10% of participants were forwarded to a shortlist. In the second round, outcomes on this shortlist were rated and prioritised. The final COS was defined in a face-to-face expert meeting of paediatricians. Results: F of invited stakeholders (133 HCPs and 55 parents of infants with IC) completed both Delphi rounds. Duration of crying, family stress, sleeping time of infant, quality of life (of family), discomfort of infant and hospital admission/duration were rated as most important outcomes in IC, framing the final COS. Conclusions: The use of this COS should serve as a minimum of outcomes to be measured and reported. This will benefit evidence synthesis, by enhancing homogeneity of outcomes, and enable evaluation of success in therapeutic trials on IC. Researchers are strongly encouraged to use this COS when setting up a clinical trial in primary, secondary and/or tertiary care or performing a systematic review on IC.