Human group 1 ILCs consist of at least three phenotypically distinct subsets, including NK cells, CD127⁺ ILC1, and intraepithelial CD103⁺ ILC1. In inflamed intestinal tissues from Crohn's disease patients, numbers of CD127⁺ ILC1 increased at the cost of ILC3. Here we found that differentiation of ILC3 to CD127⁺ ILC1 is reversible in vitro and in vivo. CD127⁺ ILC1 differentiated to ILC3 in the presence of interleukin-2 (IL-2), IL-23, and IL-1β dependent on the transcription factor RORγt, and this process was enhanced in the presence of retinoic acid. Furthermore, we observed in resection specimen from Crohn's disease patients a higher proportion of CD14⁺ dendritic cells (DC), which in vitro promoted polarization from ILC3 to CD127⁺ ILC1. In contrast, CD14⁻ DCs promoted differentiation from CD127⁺ ILC1 toward ILC3. These observations suggest that environmental cues determine the composition, function, and phenotype of CD127⁺ ILC1 and ILC3 in the gut.