A group of Swedish oncologists and hospital physicists have estimated the number of patients in Sweden suitable for proton beam therapy. The estimations have been based on current statistics of tumour incidence, number of patients potentially eligible for radiation treatment, scientific support from clinical trials and model dose planning studies and knowledge of the dose-response relations of different tumours and normal tissues. In gastrointestinal cancers, it is assessed that at least 345 patients, mainly non-resectable rectal cancers, oesophageal and liver cancers, are eligible. Great uncertainties do however exist both in the number of patients with gastrointestinal cancers suitable for radiation therapy, and in the proportion of those where proton beams may give sufficiently better results.