To describe the quality of oncology guidelines developed in different countries. The Appraisal of Guidelines and Research and Evaluation (AGREE) Instrument was used to assess the quality of 100 guidelines (including 32 oncology guidelines) from 13 countries. The criteria of the instrument are grouped into six quality domains: scope and purpose, stakeholder involvement, rigor of development, clarity and presentation, applicability, and editorial independence. Oncology guidelines had significantly higher scores on rigor of development than nononcology guidelines (42.2% v 29.4%; P =.02). In particular, systematic methods to search for evidence were more often used (P =.01); the methods for formulating the recommendations were more clearly described (P =.02); and health benefits, risks, and side effects were more often considered in formulating the recommendations (P =.03). Although the standardized scores for the other domains were not significantly different, the oncology guidelines had significantly higher scores for items measuring inclusion of all relevant professional groups (P =.05), consideration of patient views (P =.04), and presentation of different options (P =.05). Only three organizations producing oncology guidelines had standardized scores more than 60% for more than three domains. The quality of clinical practice guidelines (CPGs) is modest in general, but for certain domains, oncology guidelines seem to be of better quality than others. The experience of the organization may explain higher scores for some items. Research projects and training aimed at improving the quality of guidelines should be developed. The AGREE instrument could provide a basis for defining steps in a shared development approach to produce high-quality CPGs.