Neuroendocrine tumours of the gastrointestinal tract are relatively uncommon neoplasms with, in spite of their characteristic morphology, relatively unpredictable biological behaviour. In some sites, notably the appendix, these tumours are largely benign whereas at other localisations, such as the small bowel, metastases occur and the outcome is less favourable. Given the lack of discriminative power of histological parameters, immunohistochemical parameters have been proposed. Of these the Ki-67 index, as an indicator of proliferative activity, has shown some promise. In order to assess their proliferative activity and the potential contribution of this parameter to defining biological behaviour, we performed Ki-67 immunostaining of a series of 64 well differentiated neuroendocrine tumours of the gut (stomach, small bowel, appendix, colon and rectum). Ki-67 labeling index, based upon counting of up to 5000 cells, ranged between 0 and 6.1%. No difference was found according to age, gender, size, location or TNM classification. Ki-67 labeling index of midgut endocrine tumours of long term surviving patients did not differ from patients that died. We conclude that Ki-67 labeling index as an indicator of proliferative activity of well differentiated neuroendocrine tumours of the digestive tract does not correlate with size nor site nor stage. Even though only small numbers of tumours could be analysed, which hampered appropriate statistical analysis, it seems unlikely that proliferative activity has potential as an independent prognostic parameter for this type of tumour.