The EORTC Study Group on Quality of Life (the Study Group) has adopted a modular approach to quality of life (QOL) assessment in cancer clinical trials. A 'core' instrument-the QLQ-C30-has been designed to cover a range of QOL issues relevant to a broad spectrum of cancer patients. It is intended that this QLQ-C30 be supplemented by more specific subscales ('modules') to assess aspects of QOL of particular importance to specific subgroups of patients. Since individual members of the Study Group were to be involved in module development, guidelines were required to standardize the module development process in order to ensure uniformly high quality across modules. These guidelines are presented in this paper. The term 'module' is defined, the composition of modules is outlined, and the criteria used to develop modules are specified. The module development process, consisting of four phases (generation of relevant QOL issues, operationalization of the QOL issues into a set of items, pretesting the module questionnaire, and large-scale field-testing) is described in detail. Further, issues related to cross-cultural instrument development, and the need for monitoring the module development process from within the Study Group are discussed. Finally, experiences with developing two site-specific modules (i.e., for head and neck, and breast cancer), are presented and the extent to which the guidelines meet practical requirements is discussed. The guidelines appear to provide a practical tool for module construction, that can facilitate the development of a comprehensive system for assessing the QOL of cancer patients internationally.