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Background Innate lymphoid cells (ILCs) are a subset of innate immune cells that mimic the functions of T lymphocytes, but lack rearranged antigen receptors or clonal expansion in response to antigens. They are characterized in human peripheral blood as Lin− (CD3, CD1a, CD4, CD14, CD16, CD19, CD34, CD94, CD303, FceRI) CD127+ cells. ILCs are divided into three sub-groups: ILC type 1, 2 and 3. The recovery of these cells after allogeneic hematopoietic stem cell transplant (aHSCT) in the pediatric population has not been described. Objectives The primary objective is to study the recovery of ILCs after aHSCT in pediatric patients. The secondary objective was to determine if recovery of ILCs correlated with the incidence or severity of graft-versus-host disease (GVHD). Methods We are conducting a prospective study, enrolling consecutive patients undergoing aHSCT at our institution. After approval from local IRB, the study opened in May 2019. Blood samples are drawn from patients prior to the conditioning regimen, and then weekly starting on day 0 until day +30 (+/- 3 days except day 0), then on day +60, day +90 and day +180. All blood samples are analyzed on the same day by flow cytometry along with a healthy control (thawed frozen peripheral blood mononuclear cells [PBMCs]), serving as the positive control. Results All eligible patients have consented to the study. Six have been enrolled through September 2019; with the following demographics: median age is 9 years (range 21 months-18 years); 2 malignant, 4 non-malignant; matched sibling (n = 1), unrelated (n = 4 complete match, n=1 mismatched 9/10). All patients received donor bone marrow. Conditioning regimens were variable depending on the indication for transplant, with fludarabine, busulfan and individualized dose rATG being the most commonly utilized regimen (67%). GVHD prophylaxis for all patients was tacrolimus with mini-methotrexate. We observed substantial drop in number of ILCs in response to conditioning regimen in all patients, with nadir on day +14 that plateaued through day +30, followed by recovery by day +60 (figure 1). The one patient who has day +90 sample had a complete recovery of total number of ILCs. Similar trend was seen in type 2 ILCs (Lin− CD127+ CRTH2+) and Type 3 ILCs (Lin− CD127+ CRTH2-ve CD117+) while type 1 ILCs (Lin− CD127+ CRTH2− CD117−) showed a slower drop, reaching a nadir on day +21 with slow recovery by day +60. Interestingly, patients who had GVHD (n = 3), had a higher number of ILCs compared to patients who did not develop GVHD (n = 3) on day 0 (p = 0.0781, 2way ANOVA) (Figure 2). This pattern was statistically significant in type 1 ILCs between the two groups of patients on day 0 (p = 0.0062, 2way ANOVA) (Figure 3). Conclusion It is feasible to prospectively study the recovery of ILCs in the post aHSCT pediatric population. We plan to enroll additional patients allowing us to better evaluate a possible correlation between ILCs and GVHD.