Quality of life (QoL) measurements are increasingly being used as an end point in cancer clinical trials. Standard generic QoL questionnaires may not assess symptoms produced by neuroendocrine tumours. Here we report the development of a disease-specific quality of life score questionnaire for patients with neuroendocrine tumours of the gut to supplement the EORTC core cancer questionnaire, the QLQ-C30. Phases 1-3 of the EORTC quality of life group guidelines for module development were used to design the new questionnaire. Forty-one relevant issues (questions) were generated after an extensive literature search. Following interviews of 15 health care workers and 35 patients, a 35 question provisional questionnaire was constructed. This was translated into seven European languages and pre-tested in 180 patients resulting in a 21-item module that will be validated in an international clinical trial. The EORTC QLQ-NET21 provides a site-specific module to supplement the QLQ-C30 for patients with neuroendocrine tumours.