Since 1977, when the first edition of the Manual for Staging of Cancer was published by the American Joint Committee on Cancer (AJCC), the primary tumour staging for oral squamous cell carcinoma (SCC) has remained unchanged. In the last four decades important prognostic factors in oral cancer have been identified. Depth of invasion (DOI) of the primary tumour is one independent predictor of recurrence and survival out of many others. Depth of invasion is already a feature in the AJCC staging of other cancers such as melanoma, cutaneous SCC and the uterine cervix. In this multicentre, retrospective study, author’s pooled data from 11 participating comprehensive cancer centres worldwide. Patients with histologically confirmed oral SCC undergoing surgical resection of the primary tumour and neck dissection with curative intent were candidates for inclusion. The final study population consisted of 3,149 patients. On multivariable analysis, DOI was significantly associated with disease-specific survival (P < 0.001), and provided complementary prognostic information to the AJCC T category. The authors propose a modification that incorporates DOI in future versions of the AJCC staging.