This paper describes the translation to Castilian and adaptation of a quality of life measurement instrument: the Asthma Quality of Life Questionnaire (AQLQ). The AQLQ, developed by Juniper et al, contains 32 items, 5 of which relate to habitual activities that the patient can choose from among a list of 26 possibilities. Answers are given on a scale of 7 points. To adapt this instrument for use in Spain, we subjected it to a process of translation/back translation by bilingual informants. The translated and original versions of each item, activity and answer option were evaluated as being totally equivalent (A), fairly equivalent but with some questionable wording (B), or of questionable equivalence (C). The naturalness and correctness of the Spanish version were also evaluated on a scale of 1 to 10. Three (9%) items and 1 (4%) activity were considered to be of questionable equivalence (C) and 12 (37%) items and 1 activity (4%) were considered to be of type B equivalence. The questionable aspects of types B and C equivalence were discussed in 2 meetings, along with expressions that were equivalent but unnatural or grammatically incorrect; the first meeting involved researchers and translators and the second was held with a group of 6 asthmatics. Consensus was finally obtained for each item and activity included in the second draft. That draft was then administered to another group of 7 patients in order to check comprehension and equivalence, after which a definitive version was produced by the researchers.(ABSTRACT TRUNCATED AT 250 WORDS)