Several recent studies have indicated that cells at the invasive tumour margins often are different from cells within other parts of various human cancers. In this work, we have studied all squamous cell carcinomas of the floor of the mouth registered in Norway during the years 1963-1972 (N = 96). Borderline cases and cases given no treatment were excluded. Of the remaining 79 cases, biopsy specimens acceptable for histological grading were obtained from 61 patients. Only the most invasive margins of the tumours were histologically graded independently by two pathologists according to a multifactorial grading system. The results confirmed our previous findings that grading of invasive tumour margins is an independent prognostic factor in Cox's multivariate survival analysis (P less than 0.01). Inter-observer agreement was calculated by kappa statistics, and good agreement was obtained (kappa = 0.63). Neither agreement nor prognostic value was improved after calibration of the pathologists. Conventional Borders' grading of the whole biopsy had no prognostic value (P less than 0.38). We conclude that invasive cell grading may be of value for treatment planning of oral cancers, and that further studies of the deep, invasive parts of oral and other cancers are needed in order to obtain a better understanding of tumour cell invasion and metastasis.