The aim of the study is to compare the performance of the Juniper Asthma Quality of Life Questionnaire (AQLQ) and the St George's Respiratory Questionnaire (SGRQ) in a sample of asthmatic patients, representative of a broad spectrum of asthma severity. We studied 116 patients with a mean age (SD) of 42.6 (18.3) year. Patients were assessed twice, at recruitment and after 2 months, to determine the reliability, validity and responsiveness of the AQLQ and the SGRQ. Both questionnaires showed good reliability coefficients (> or = 0.70) which reached the standards for comparison at individual level (> or = 0.90) in the case of activity, impacts and overall SGRQ scores as well as symptoms, activities and overall AQLQ scores. Both AQLQ and SGRQ were able to discriminate among groups of patients based on asthma severity and control and showed, except for the symptoms domain of the SGRQ, large (standardized response means >0.8) and significant changes in the group of patients that improved at follow-up. We conclude that the AQLQ and SGRQ have shown high reliability and validity and, with the exception of the SGRQ symptoms, a high level of responsiveness. In overall terms, not one of these instruments seems to behave better than the other.