We analysed 32 primary, 8 recurrent and 16 metastatic squamous cell carcinomas of the head and neck by flow cytometry to assess tumoural heterogeneity. Intratumoural heterogeneity was found in 9 of 30 primary tumours (30%). All heterogeneous tumours had diploid and aneuploid cell lines. DNA indices were different from those of the primary tumours in 3 of the 8 recurrent lesions (38%) and in 8 of the 12 metastatic lesions (67%). The results indicate that new cell lines evolve during the process of recurrence and metastasis. The incidence of recurrence, metastasis, and heterogeneity increased with the size of the tumour. Histologically, there were tumours in which the degree of differentiation and mode of invasion of primary lesions were not consistent with the results of flow cytometry in both homogeneous and heterogeneous tumours. In conclusion, multiple sampling for flow cytometric analysis is essential for better characterization of oral carcinomas.