Summary:Toxoplasmosis in bone marrow transplant recipients is a rare but serious complication and if untreated, almost uniformly fatal. The diagnosis, however, remains difficult. We therefore compared serial determination of antibody titers specific for T. gondii before and after transplantation, serial PCR for T. gondii DNA in serum, PCR and nested PCR for T. gondii DNA in various tissues, conventional histology and immunohistochemistry for detection of parasites in three patients with autopsy-confirmed toxoplasmosis after bone marrow transplantation. Immunohistochemistry demonstrated the presence of parasites in 13 out of 20 organs investigated (65%), whereas PCR detected T. gondii-specific DNA in 15 out of 20 organs (75%). Immunohistochemistry revealed concordant results to PCR data in 60% of the specimens. With the use of a nested PCR protocol, eight out of nine samples (89%) were positive for T. gondiispecific DNA. The combination of both methods detected the presence of parasites in 90% of the specimens. Serial PCR in serum did not yield positive results. Neither PCR nor immunohistochemistry was able to detect parasites in all organs investigated, but both methods together improved sensitivity to 90% and consequently, should be used jointly to maximize diagnostic precision. Bone Marrow Transplantation (2000) 25, 1257-1262. Keywords: toxoplasmosis; bone marrow transplantation; PCR; immunohistochemistry; serology Toxoplasma gondii is a protozoon affecting and persisting in almost all known mammals. In man, it is known to invade many cell types causing an asymptomatic infection in the vast majority of immunocompetent individuals. Toxoplasmosis results from reactivation of latent infection when the individual is immunocompromised. The incidence of toxoplasmosis and seropositivity varies considerably between different countries and within distinct geographic regions, depending on local pet habits, especially the presence of cats and food contamination with cysts.