Toxoplasmosis in allo-SCT patients is rare in the United States but has a mortality of 60-90%. In this retrospective study, we identified patients with definite and probable toxoplasmosis after allo-SCT at our institution from 1994 to 2009 using ICD-9 codes and the pathology database. Of 3626 patients who underwent allogeneic SCT, we identified 8 with definite toxoplasmosis and 1 with probable toxoplasmosis, an incidence of 0.25%. International patients had a significantly higher incidence of toxoplasmosis than did US patients (1.6 versus 0.15% (P=0.002)). Three patients presented with toxoplasmosis <30 days after transplantation and six developed toxoplasmosis within 100 days of starting high-dose corticosteroids. Two patients were diagnosed postmortem. Six of the remaining seven patients died despite ≥2 weeks of therapy. Co-morbidities, particularly infections, were the primary cause of death in one case and a contributing factor in the remaining six cases. On the basis of these results, we conclude that all allo-SCT patients from countries with high Toxoplasma seropositivity and seropositive patients from the United States should undergo serial PCR screening during the first month after transplantation and during corticosteroid use. All patients who test positive should undergo preemptive therapy.