To analyse initial tobacco-related cellular alterations in the upper aerodigestive tract, we investigated the proliferation state in paraffin embedded samples of tumour-adjacent histologically normal mucosa from head and neck squamous cell carcinoma (HNSCC) patients and normal buccal mucosa from healthy individuals. The proliferation index (PI) was assessed by indirect immunohistochemical staining for the proliferation marker Ki-67. Only a slight rise in PI was seen in the normal epithelium from non-smoking HNSCC patients in comparison with the epithelium from non-smoking healthy individuals. The epithelia from the smoking HNSCC patients and from the healthy smoking individuals both showed an increased PI compared with epithelia from the non-smoking HNSCC patients and healthy individuals (P = 0.001). In addition, the ex-smokers in both groups still showed a trend towards increased PI. Increased PI after cessation of smoking could indicate permanent epithelial alterations. Our findings provide new evidence for the concept of field cancerisation.