Acute tonsillitis is usually caused by a viral infection, however often antibiotics are prescribed without a confirmed diagnosis of a bacterial infection. The Modified Centor (McIsaac) criteria, which include fever (>38°c), tonsillar exudate, no cough, anterior cervical lymphadenitis and age, can be used to determine group A Streptococcus infection in acute tonsillitis cases. In this Turkish study, 97 patients diagnosed with acute tonsillitis were analysed and the McIsaac scores were compared to the patients’ throat cultures, white blood cell counts (WBC), C-reactive protein (CRP) levels, neutrophil counts, erythrocyte sedimentation levels. The patients were separated into two groups, (group 1 for McIsaac scores of ≤2 with 29 patients, and group 2 for McIsaac scores of ≥3 with 68 patients). There was a statistically significant correlation between the McIsaac scores of group 2 patients and the presence of group A Streptococcus infection on throat swabs (p < 0.05). CRP, WBC and total neutrophil counts were significantly higher in patients with throat cultures positive for group A Streptococcus infection than in those with negative throat cultures. This study suggests that the McIsaac score can assist in stratifying those patients more likely to have a bacterial infection requiring antibiotics, which may aid in the prevention of unnecessary antibiotic use.