This study aimed to assess the frequency and severity of adverse drug reactions (ADRs) (WHO definition) in hospitalized adult patients in an infectious diseases referral ward in Tehran. Of 281 patients evaluated over 6 months, a total of 170 suspected ADRs were reported among 101 patients (35.9%). The most commonly affected organ system was gastrointestinal (47.5%), and the most common class of drugs responsible was anti-infectives (93.1%). ADRs were high among HIV-positive patients (82.9%), mainly due to anti-tuberculosis drugs. Attention to appropriate prescription of drugs is required with more careful clinical and laboratory monitoring of patients.