Extraction of two upper premolars in Class II division 1 occlusions often constitutes a therapeutic compromise for the orthodontic practitioner. The aim of our study was to compare the initial occlusal and cephalometric severity of Class II division 1 malocclusions in two groups of patients treated with and without extraction of two upper premolars and thus determine the factor or factors determining this therapeutic option. Examination of the casts and cephalometric analysis of 31 patients presenting a Class II division 1 malocclusion were made. The non-extraction group comprised 16 patients and the group undergoing extraction of two upper premolars comprised 15 patients. Discriminant analysis was applied using binary decision trees in order to identify the variable which best distinguished the two groups. Maxillary incisor-canine crowding was selected to discriminate between the patients at pretreatment stage; 93.5% of the patients were correctly classified using this factor.