OBJECTIVE: To find a quickly available screening tool for the differentiation of patients with glandular fever from those with acute purulent tonsillitis. The null hypothesis was that there was no difference between the lymphocyte-white blood cell count (L/WCC) ratio between the 2 patient groups. DESIGN: Retrospective pilot study based on laboratory tests for lymphocyte counts, white blood cell counts, and the mononucleosis spot test. SETTING: Ear, Nose, and Throat Department, St George's Hospital, London, England. PATIENTS: One hundred twenty patients with glandular fever and 100 patients with bacterial tonsillitis. MAIN OUTCOME MEASURES: Results from the mononucleosis spot test in conjunction with the clinical picture and the L/WCC ratio were analyzed. Significant differences were evaluated using the Mann-Whitney test and Fisher exact test. RESULTS: The L/WCC ratio was significantly different in the 2 groups (P<.001). The mean L/WCC ratio in the glandular fever group was 0.54 and the mean L/WCC ratio in the bacterial tonsillitis group was 0.10. A ratio higher than 0.35 had a specificity of 100% and a sensitivity of 90% for the detection of glandular fever. CONCLUSIONS: We recommend that the L/WCC ratio should be used as an indicator to decide whether mononucleosis spot tests should be requested. A ratio higher than 0.35 had a high specificity in our study group.