Toxoplasmosis is a parasitic disease associated with high mortality in immunocompromised patients. It may lead to life-threatening conditions, usually neuroinfections, pneumonia or disseminated disease. It may be potentially dangerous, especially for patients with prolonged lymphopenia or those treated with immunosuppressive drugs. In our centre, we have observed 3 cases of toxoplasmosis in patients after allogeneic haematopoietic stem cell transplantation (HSCT) (2.6% of 116 allo-HSCT patients since 2000) and one case after autologous HSCT (0.3% of 395 auto-HSCT patients since 1997). Toxoplasmosis is manifested by neurological symptoms including hemiparesis and paraparesis, cerebral salt-wasting syndrome (hyponatraemia and hypoosmolality), psychoorganic syndrome and signs of respiratory infection. The diagnosis was made by combining clinical signs and results of PCR and CT examinations. The patients were treated with high-dose pyrimethamine, clindamycin, co-trimoxazole and folic acid. Three of the four patients have survived with no signs of the disease. One patient died prior to treatment. The increasing use of highly immunosuppressive chemotherapy and conditioning regimens (including rituximab, fludarabine and anti-thymocyte globulin) is associated with a significant risk of toxoplasmosis. Variable manifestations, non-specific results of MRI or CT examinations and possibility of PCR negativity are the main obstacles to successful diagnosis.