Out of a consecutive series of 698 cases of squamous cell carcinomas of the tonsillar region treated by radical megavoltage radiotherapy, a determinate group of 465 cases remained eligible for a multivariate analysis of the pretreatment features of the disease and treatment-related parameters predictive of lasting control of the disease at the primary site. T-stage and initial site within the tonsillar region, were the significant pretreatment factors. Tumours arising from the glossopalatine sulcus which are characterized by involvement of the tongue, do significantly worse than those arising from other sites within the tonsillar region: i.e. the tonsil itself, posterior pillar and to a lesser extent the anterior pillar. As regards treatment-related parameters in the 465 cases which received tumour doses of at least 55 Gy, only the length of overall treatment time was found to be predictive. Combining both pretreatment and treatment variables, T-stage (p less than 0.0001), overall treatment time (p less than 0.0001) were by decreasing order of significance the predicting factors, followed by initial site (p = 0.006). When present, tumour extension to the anatomical structures anterior to the tonsillar region was also found to be significant (p = 0.05). Based on these factors, a multivariate model was constructed and tested by estimating the product-limit survival of the various categories of patients. Four groups are individualized with 3 years local control rates ranging from 90 to 21%. The predictive accuracy of the model was assessed by log-rank test significance levels. The model may help to select patients for whom conventional radical radiotherapy is inadequate, and combined modality or altered fractionation regimes should be tried particularly for advanced tumours of the glossopalatine sulcus, and any case with significant invasion of the oral cavity.