Forty-one cases of toxoplasmosis were diagnosed in 15 European transplantation centers in patients who had undergone allogeneic hematopoietic stem cell transplantation (HSCT) from 1994 through 1998. Most patients (39 [94%]) were seropositive for Toxoplasma gondii before they underwent transplantation, and 30 (73%) had developed moderate to severe acute graft-versus-host disease before they developed toxoplasmosis. Thirty-five (85%) patients had Toxoplasma disease with evidence of organ involvement, whereas 6 (15%) patients had Toxoplasma infection, as defined by fever and a positive polymerase chain reaction (PCR) finding for T. gondii in blood. Nine patients were diagnosed at autopsy. Thirty patients (73%) had not received antimicrobial prophylaxis with anti-Toxoplasma activity after undergoing transplantation. The median day of onset of disease after HSCT was 64. Twenty-two (63%) patients died from toxoplasmosis, and 23 (66%) received adequate anti-Toxoplasma therapy for > or =3 days. Among these 23 patients, 11 (48%) showed a complete response and 3 (13%) showed improvement. In univariate and multivariate analyses, having received adequate therapy and experiencing late infection (>63 days after HSCT) were associated with a lower risk of dying from toxoplasmosis. Toxoplasmosis after HSCT is a severe infection with a high mortality rate even when diagnosed soon after HSCT, and PCR may help establish the diagnosis earlier.