OBJECTIVES: To build on recent discussion within the ISPOR QoL/SIG group and develop methodology for the translation and linguistic validation of patient diaries. There is a considerable body of literature in support of a rigorous methodology for the translation and linguistic validation of PROs. Typically this methodology involves forward translation, back translation, developer review and pilot testing, with ongoing harmonisation between translated versions. This methodology aims to achieve linguistic equivalence - essential if data from multinational trials are to be pooled. Recent discussion within the ISPOR QoL/SIG group has focused on whether this same methodology should also be applied to patient diaries. Patient diaries are designed to be completed at time intervals, and thus erroneous information or wording might be repeated, something unlikely to occur in other PROs. METHODS: Relevant literature on translation and linguistic validation of PROs was reviewed. Several translated patient diaries were analysed for their content and for concepts that proved problematic in translation. The results were discussed and form the basis of this paper's findings and recommendations. RESULTS: A number of translation difficulties were encountered with patient diaries. These can be broadly categorized as follows: a) psychological concepts regarding patient thoughts and feelings; b) clinical information, e.g. country-specific names for drugs; c) references to country-specific healthcare and welfare systems; and d) miscellaneous linguistic ambiguities. Involvement of psychological concepts indicates a need for pilot testing with patients. The presence of clinical information and references to country-specific healthcare and welfare systems indicates that clinical personnel should be involved in translation. CONCLUSIONS: A concept elaboration stage, to review diary content and identify translation difficulties, is needed before a translation methodology is selected for patient diaries. This approach will lead to the selection of appropriate methodology and result in a translation that is faithful to the concepts within the source language.