With the increase of QoL assessment in clinical trials, it is necessary to have cross-culturally valid instruments to make comparisons of health status outcomes and pool data across countries. The IIEF is a 15-item instrument designed in English to assess erectile dysfunction. Prior to use in an international trial, it had to undergo cross-cultural adaptation in 31 languages. This involved the recruitment of a QoL specialist in each target country. Two independent forward translations were produced by native speakers, bilingual in English. These were reconciled and backtranslated into English. The translations were tested for comprehension in a sample target population, compared and internationally harmonised. The developer clarified concepts underlying each item. Translation was difficult, especially when selecting equivalent response categories and appropriate equivalents of common English structures and expressions. The difficulty of the translation and its acceptability depended on the comparability of the original and the target language and culture. As English is flexible, it was possible to accept "most of the times (much more than half of the time) as an answer to the question: "how often have you felt sexual desire in the last 4 weeks)". In many languages a literal translation would have been unclear and was replaced by "how much of the time...". Patient testing showed different reactions to the instrument across countries and made it clear that terms such as "intercourse", "erection" or "ejaculation" were not always understood and required explanation. Prior to use in an international trial, rigorous cultural adaptation was essential to producing cross-culturally valid language versions. Psychometric testing will be important to ensure similar relationships among scales across countries. The comparison of answer patterns across countries will clarify whether people with similar health give equivalent answers.