In previous studies, we and others observed in patients undergoing HLA-matched hematopoietic cell transplantation that high proportion of donor-derived CD4+/CCR7+ T cells were associated with an increased risk of acute GVHD without any interference in relapse incidence. We investigated the impact of donor-derived CD4+/CCR7+ T cells on patient outcome in haploidentical settings where posttransplant cyclophosphamide is used. We analyzed T-cell subsets in grafts of 29 adult patients who underwent first haploidentical transplant following reduced intensity conditioning. The median CD4+/CCR7+ subset proportion was 69.2% among donor CD4+ T cells. With a median follow-up of 28.1 months (range: 11.0–44.3), 16 patients (55%) developed acute GVHD; this includes 5 patients with grade 3 acute GVHD. Fifty-four percent of patients who received > 69.2% of CD4+/CCR7+ T cells and 12% of patients who received < 69.2% CD4+/CCR7+ T cells developed acute GVHD (p = 0.028). In multivariate analysis, a high proportion of CD4+/CCR7+ T cells was the only factor that impacted acute GVHD (HR = 4.925, 95% CI [1.020–23.775], p = 0,047) with no impact on overall survival. Our results confirm the impact of a high proportion of CD4+/CCR7+ T cells on acute GVHD incidence in patients undergoing haploidentical transplant despite the use of posttransplant cyclophosphamide.