BACKGROUND: Good syndrome (GS) refers to a constellation of thymoma and adult-onset immunodeficiency characterized by low or absent B cells, hypogammaglobulinemia, and variable defects in cell-mediated immunity with an inverted CD4/CD8(+) T-cell ratio. Patients may develop severe or chronic infections as a result of this immunodeficiency. OBJECTIVE: To describe a patient with GS who developed cytomegalovirus (CMV) encephalitis and showed a periodic electroencephalographic pattern and myoclonus. DESIGN: Case report. SETTING: Outpatient neurology clinic at a university medical center. Patient A 64-year-old man who developed periodic myoclonus involving the right half of his body and the left arm. RESULTS: Five years previously, the patient had undergone resection of a pathologically confirmed epithelial thymoma. Quantitative CMV polymerase chain reaction of the serum and cerebrospinal fluid showed strongly positive results (1:10 000), allowing the diagnosis of CMV encephalitis. CONCLUSIONS: To the best of our knowledge, myoclonus and periodic electroencephalographic pattern have not previously been reported in CMV encephalitis. Opportunistic CMV infection should be considered early in the evaluation of patients with GS or a history of thymoma who develop unusual neurological symptoms.