The aim is to determine if tumor thickness is a risk factor related to the development of early recurrences in surgically treated oral cavity squamous cell carcinoma (SCC). Retrospective cohort study conducted at Instituto do Câncer do Estado de São Paulo (ICESP). Fifty-seven patients with oral cavity SCC (excluding lip tumors and patients previously submitted to any treatment) were analyzed regarding the occurrence of an early disease progression (locoregional or distant metastasis) within the first 12 months after initial treatment. Tumor thickness and other histological characteristics related to the development of recurrence up to 1 year after treatment were tested. Results demonstrated that tumor thickness greater than 10 mm (P = 0.034), as well as angiolymphatic invasion (P = 0.001), perineural invasion (P = 0.041) and lymph-node metastasis (P = 0.021) was associated with a worse 12-month disease-free survival (Log-Rank test). In multivariate analysis, tumor thickness greater than 10 mm emerged as an independent risk factor for early recurrence in oral cavity tumors (HR = 3.4, 95% CI: 1.005-11.690; P = 0.049--Cox regression). Post-operative radiotherapy seems to be a protective factor for early recurrences in patients with tumor thickness greater than 10 mm (P = 0.017--Log-Rank test; HR = 0.32, 95% CI: 0.12-0.87, P = 0.026--Cox regression). The results of the present research suggest that tumor thickness greater than 10 mm may be an independent adverse factor for early progression of surgically treated oral cavity SCC. Adjuvant therapies, in particular post-operative radiotherapy, should be advocated in this group of patients, regardless of the co-existence of other well-described histological risk factors.