We report a 60-year-old woman with toxoplasma encephalopathy. She was being treated with prednisolone and methotrexate for rheumatoid arthritis that was diagnosed at the age of 40. In a preoperative examination of her left fifth finger ganglion, pericardial effusions, cardiomegaly, and a right atrial mass were detected. In addition, brain MRI showed nodular shadows in the right thalamus, bilateral globus pallidus, and left dentate nucleus of the cerebellum. T1 and T2 weighted images showed high intensities within those shadows; however, a T1 gadolinium enhancement image showed no contrast enhancement in the lesions. There were no positive neurological findings. Examination of the cerebrospinal fluid and cultivation tests showed nothing particular. The right atrial mass was subsequently diagnosed as malignant lymphoma and treated with radiation therapy. Toxoplasma gondii antibody titers were increased in both serum and cerebrospinal fluid. Based on IgG avidity index and nested PCR, we diagnosed toxoplasma encephalopathy with chronic T. gondii infection. The T. gondii gene product was also detected in cerebrospinal fluid by nested PCR. We consider that IgG avidity index and nested PCR were useful for the diagnosis of toxoplasma encephalopathy.