BACKGROUND: The Patient-Reported Outcomes Measurement Information System (PROMIS(®)) is a new, state-of-the-art assessment system for measuring patient-reported health and well-being of adults and children that has the potential to be more valid, reliable and responsive than existing PROMs. The PROMIS items can be administered in short forms or, more efficiently, through computerized adaptive testing. This paper describes the translation of 563 items from 17 PROMIS item banks (domains) for adults from the English source into Dutch-Flemish. METHODS: The translation was performed by FACITtrans using standardized methodology and approved by the PROMIS Statistical Center. The translation included four forward translations, two back-translations, three to five independent reviews (at least two Dutch, one Flemish) and pre-testing in 70 adults (age range 20-77) from the Netherlands and Flanders. RESULTS: A small number of items required separate translations for Dutch and Flemish: physical function (five items), pain behaviour (two items), pain interference (one item), social isolation (one item) and global health (one item). Challenges faced in the translation process included: scarcity or overabundance of possible translations, unclear item descriptions, constructs broader/smaller in the target language, difficulties in rank ordering items, differences in unit of measurement, irrelevant items or differences in performance of activities. By addressing these challenges, acceptable translations were obtained for all items. CONCLUSION: The methodology used and experience gained in this study can be used as an example for researchers in other countries interested in translating PROMIS. The Dutch-Flemish PROMIS items are linguistically equivalent. Short forms will soon be available for use and entire item banks are ready for cross-cultural validation in the Netherlands and Flanders.