OBJECTIVES: To develop a practical and scientifically rigorous, patient-based outcome questionnaire to evaluate symptoms in community-acquired pneumonia (CAP). METHODS: The CAP-Symptom questionnaire (CAP-Sym) is an 18-item, interviewer-administered questionnaire that measures the bothersomeness of 18 symptoms during the past 24 hours using a 6-point Likert scale. We used gold-standard psychometric methods to comprehensively evaluate the acceptability, reliability, validity and responsiveness of the CAP-Sym in field testing involving 556 patients in 13 countries. The development and validation of the CAP-Sym were carried out as part of the CAP 2000 study, a multicentre, multinational, prospective, randomised, double-blind study to compare the effectiveness of moxifloxacin oral tablets to standard oral treatment regimes in patients with CAP. RESULTS: Field testing in all countries confirmed the acceptability (item non-response, item endorsement frequencies, item/scale floor and ceiling effects), reliability (internal consistency, item-total and inter-item correlations, test-retest reliability), validity (content, construct, convergent, discriminant, known groups) and responsiveness of the CAP-Sym. CONCLUSIONS: The CAP-Sym is a practical and scientifically sound patient-based outcome measure that can be used to evaluate CAP-related symptoms in clinical trials or clinical audit. The disease-specific CAP-Sym shows preliminary evidence of being more responsive than the generic SF-36 as a measure of outcome in CAP.