We report an unusual case of cerebral toxoplasmosis associated with Guillain-Barré syndrome (GBS) in a 25-year-old patient diagnosed with chronic myelogenous leukaemia (CML), who underwent a mismatched allogeneic peripheral stem cell transplantation (PSCT). On day +83 he started with fever, and 7 days later tremor, muscular weakness, diplopia, dysarthria, respiratory difficulty, and universal arreflexia appeared, compatible with GBS. As the patient had a positive cytomegalovirus (CMV) antigenemia, this was the aetiology suspected for his neurologic findings, but specific treatment failed to improve his clinical situation, and he died on day +123. Necropsy demonstrated cerebral toxoplasmosis and axonal degeneration of nerve roots compatible with the axonal form of GBS. Interestingly, the polymerase chain reaction (PCR) signal for Toxoplasma gondii in two different cerebrospinal fluid (CSF) samples had been negative. In addition, this case showed unique magnetic resonance imaging (MRI) abnormalities. We conclude that a negative PCR on CSF cannot exclude toxoplasmosis in a transplant patient, and we emphasise the importance of considering Toxoplasma as an aetiology of fever and neurological symptoms in the transplant setting.