Objectives: The aim of the present study was to assess the knowledge and attitude of patients with gender identity disorder (GID), presenting to Tehran Psychiatric Institute, toward Acquired immunodeficiency Syndrome (AIDS).  Method: 58 patients with gender identity disorder (41 female-to-male and 17 male-to-female) were selected using convenience sampling. The subjects completed the questionnaire proposed by Family Health Organization. Subsequent to determining the distribution by statistical methods, data were analyzed using parametric and non-parametric tests suitable for group comparison. Results: About 70% of the subjects believed that an apparently healthy individual could be infected with human immunodeficiency virus (HIV). Only five subjects (8.6%) considered themselves to be at risk for HIV infection. With regard to routes of transmission, 70% believed anal sex carries lower risk than vaginal sex. Conclusion: Subjects had a low level of knowledge regarding sexually transmitted diseases. Therefore, all individuals with GID should be provided with sufficient information about AIDS and sexually transmitted diseases.