This study seeks to evaluate an algorithm for the workup of HIV infected patients with focal brain lesions so as to expedite the diagnosis and subsequent treatment of PCNSL. The goals of the study are to 1) determine the specificity, sensitivity, and positive predictive value of a diagnostic algorithm that entails the use of the combination of EBV detection in the CSF and FDG-PET scanning to diagnose PCNSL; 2) evaluate the time to response to anti-toxoplasmic encephalitis therapy, and 3) evaluate the sensitivities and specificities of FDG-PET and (201)Tl-SPECT scanning in identifying PCNSL. Up to one hundred HIV-infected patients with history of at least one focal brain lesion will be screened for enrollment. All patients will be treated empirically for toxoplasmic encephalitis until an alternative diagnosis is confirmed. All enrolled patients will be treated concurrently with antiretroviral therapy. Patients identified to have PCNSL will be referred to the NCI Treatment of PCNSL Protocol for further treatment if the study is open for enrollment.