A 62-year-old man was admitted to our hospital due to fever and systemic lymph-node swelling on June 9, 1998. He was given a diagnosis of non-Hodgkin's lymphoma (diffuse mixed T cell lymphoma) with involvement of mediastinum and para-aortic lymph nodes, pleura, peritoneum, and bone marrow. After diagnosis, combined chemotherapy (CHOP regimen) was started. On day 11, the patient's consciusness level suddenly deteriorated. A brain computed tomographic scan showed no abnormal findings. A cytospin smear of cerebrospinal fluid disclosed tachyzoides (Toxoplasmic bodies), thus yielding a diagnosis of Toxoplasma encephalitis. Oral sulfadoxine pyrimethamine rapidly alleviated the patient's consciousness disturbance. One month later, a cytospin smear and RT-PCR analysis of cerebrospinal fluid detected no tachyzoides. Complete remission of lymphoma was achieved after 2 courses of CHOP therapy. Toxoplasma encephalitis should be considered a potential cause of consciousness disturbance in immunocompromised hosts, such as patients with malignant lymphoma. Cytospin smears of cerebrospinal fluid are a useful tool for the early diagnosis of Toxoplasma encephalitis.