Introduction: Anecdotally, infectious mononucleosis is considered a more severe infection than bacterial tonsillitis, requiring a longer hospital stay. However, there is little in the literature comparing the epidemiology of the two conditions. This study aimed to compare the epidemiology of bacterial tonsillitis and infectious mononucleosis, in particular any differences in the length of in-patient stay.Methodology: The hospital in-patient enquiry system was used to analyse patients admitted with bacterial tonsillitis and infectious mononucleosis between 1990 and 2009 inclusive.Results: There was a total of 3435 cases over the 20 years: 3064 with bacterial tonsillitis and 371 with infectious mononucleosis. The mean length of stay was 3.22 days for bacterial tonsillitis and 4.37 days for infectious mononucleosis. The median length of stay for each condition was compared using the Mann-Whitney U non-parametric test, and a significant difference detected (p < 0.001).Conclusion: Patients with infectious mononucleosis have a significantly longer stay in hospital than those with bacterial tonsillitis.