The treatment of advanced-stage primary cutaneous T-cell lymphomas remains challenging. Especially, large-cell transformation of mycosis fungoides is associated with a median overall survival of 2 years for all stages taken together. Little is known regarding allogeneic hematopoietic stem cell transplantation in this context. We performed a multicentre retrospective analysis of 37 cases of advanced-stage primary cutaneous T-cell lymphomas treated with allogeneic stem cell transplantation, including 20 (54%) transformed mycosis fungoides. Twenty-four patients (65%) had stage IV disease (for mycosis fungoides and Sezary syndrome) or disseminated nodal or visceral involvement (for non epidermotropic primary cutaneous T-cell lymphomas). After a median follow-up of 29 months, 19 patients experienced a relapse, leading to a 2-year cumulative incidence of relapse of 56% (95% CI, 0.38-0.74). Estimated 2-year overall survival was 57% (95% CI, 0.41-0.77) and progression-free survival 31% (95% CI, 0.19-0.53). Six of 19 patients with a post-transplant relapse achieved a subsequent complete remission after salvage therapy, with a median duration of 41 months. A weak residual tumor burden before transplantation was associated with increased progression-free survival (HR=0.3, 95%CI, 0.1-0.8, p=0.01). The use of antithymocyte globulin significantly reduced progression-free survival (HR=2.9, 95%CI, 1.3-6.2, p=0.01) but also transplant-related mortality (HR=10-7, 95%CI, 4.10-8-2.10-7, p<0.001) in univariate analysis. In multivariate analysis, the use of antithymocyte globulin was the only factor significantly associated with decreased progression-free survival (p=0.04). Allogeneic stem cell transplantation should be considered in advanced-stage primary cutaneous T-cell lymphomas, including transformed mycosis fungoides.