Objective: Acute tonsillitis is primarily caused by viral infection. Group A Streptococcus (GAS), however, is the only causative agent of acute tonsillitis for which there is a specific diagnosis and treatment. Nevertheless, antibiotics are commonly prescribed without a confirmed diagnosis. McIsaac criteria can be used to confirm GAS-associated acute tonsillitis. Herein, we aimed to study McIsaac criteria with throat cultures, procalcitonin (PCT) and C-reactive protein (CRP) levels, white blood cell counts (WBC), neutrophil counts and erythrocyte sedimentation values and to investigate utility of McIsaac criteria in the definitive diagnosis and treatment of acute tonsillitis.