Eighty consecutive patients with advanced head and neck cancer were treated with combined therapy of radical surgery and post-operative radiotherapy. Indications for post-operative radiotherapy were pathological staging of the primary tumour (pT3 or pT4), poorly differentiated or undifferentiated carcinoma (G3, G4), tumour thickness, tumour invasion in the surrounding tissues with slender tumour strands and solitary tumour cells, perineural spread, lymphangio-invasive tumour growth, multiple positive neck nodes or extranodal spread, and microscopical irradicality of the surgical margins. The contribution for prognosis of these indications for post-operative radiotherapy were retrospectively calculated in an univariate and multivariateanalysis. Of all investigated parameters, the mode of tumour invasion and lymphangio-invasive growth were independent prognostic factors. If these unfavourable prognostic signs are present, post-operative radiotherapy has to be intensified to at least curative doses of 66 Gy or more to the areas at risk.