Background and Objectives: Toxoplasmosis has become one of the more frequent opportunistic infections and the most common cause of focal brain lesions complicating the course of AIDS. However, there is a little information about the frequency of the toxoplasmosis in various regions of Iran. At the present study, we evaluated the prevalence of toxoplasmosis in patients with AIDS in Mashhasd (Northeast Iran). Material and Methods: Overall, 258 suspected patients with CD4+ T-cell count and clinical manifestation for HIV infection were referred from Infectious Disease Center to Imam Reza Hospital in 2009. These patients were examined by ELISA, western blot method and tested by P24 antigen. HIV positive patients (n=121) were entered in this cross-sectional study and investigated for IgG and IgM anti- Toxoplasma antibodies with ELISA. Results: The mean age of the patients was 35.83 + 6.75 yr. 83.5% of the patients were intravenous drug misuse. The IgG anti-Toxoplasma antibodies were positive in 46 (38.01%) patients, while IgM antibody was detected in 3 cases (2.5%). Conclusion: The rate of toxoplasmosis infection in HIV positive patients is high. Therefore, immediate treatment of these patients is essential in rising specific antibody cases that may cause toxoplasmic encephalitis resulting from its reactivation.