BiCNU (carmustine), etoposide, Ara-C, melphalan (BEAM) and Campath conditioning was developed to reduce the high transplant-related mortality in patients with lymphoma while delivering intensive antilymphoma immunotherapy, as well as to some extent a platform for allogeneic stem cell engraftment. Significant numbers of patients appeared to have persistent recipient-derived hematopoiesis, and therefore we retrospectively analyzed patients with lymphoma undergoing BEAM-Campath conditioned allogeneic stem cell transplantation at our center (2003 to 2017) to characterize the patterns of chimerism and patient outcomes. Chimerism was analyzed with short tandem repeat PCR. Mixed donor-recipient chimerism (MDRC) was defined as 5% to 94.9% donor. Fifty-two patients (n = 30 male), with a median age of 45 years, were identified with histologic diagnoses of Hodgkin lymphoma (n = 13), diffuse large B cell lymphoma (n = 7), low-grade non-Hodgkin lymphoma (n = 16), mantle cell lymphoma (n = 10), and T cell lymphoma (n = 6). Pretransplant, 93% achieved complete response (52%) or partial response (41%) with a median of 3 prior therapies (n = 3 prior autologous stem cell transplantation). Donors were Matched sibling donors (MSD) (n = 21), matched unrelated donors (MUD) (n = 24), miss-matched unrelated donors (MMUD) (n = 6), and syngeneic (n = 1). Acute graft-versus host disease (GVHD) developed in 52% (81% grade I to II) and chronic GVHD (83% extensive) in 12%. MDRC of T cells (MDRCt) developed in 62% (n = 32), and 29% (n = 15) developed MDRC of myeloid cells (MDRCm) at a median onset of 100 days. Donor lymphocyte infusion (DLI) was given to 17 patients, with a median starting dose of 1 × 106/kg. The first DLI was given at a median of 225 days post-transplant (range, 99 days to 5.3 years). Of these, 9 developed acute post-DLI GVHD and 2 limited chronic GVHD. Conversion to full donor occurred in 47% MDRCt and 50% MDRCm. Multivariate analysis identified sibling donor type as associated with increased MDRCt (P = .035; hazard ratio [HR], 0.17) and reduced total nucleated cell dose with increased MDRCm (P = .021; HR, 0.76). The median follow-up was 6 years, and 2-year NRM cumulative incidence was 16% (95% confidence interval [CI], 7% to 27%). Ten-year progression and extensive GVHD-free survival was 45% (95% CI, 28% to 61%), and overall survival was 66% (95% CI, 50% to 78%). One-year landmark analysis identified no increased GVHD or relapse risk with MDRCt or MDRCm but reduced nonrelapse mortality (NRM) risk with MDRCt (P = .001). BEAM-Campath allografts for high-risk lymphoma achieve long-term disease-free survival with low rates of GVHD and transplant-related mortality. The frequent development of myeloid MDRC demonstrates that BEAM-Campath is a nonmyeloablative conditioning regimen in almost a third of patients. MDRCt is associated with reduced NRM, but neither MDRCt or MDRCm is associated with increased GVHD or relapse.