Objectives: To design a Spanish questionnaire to assess quality of life in chronic obstructive sialadenitis (COS), named CSOC and assess its reliability, validity and feasibility. Design: A prospective, multicentre, observational study Settings: Salivary Gland Disorder and Sialendoscopy Unit, University Tertiary Hospital. Participants: Patients with diagnosis of COS and indication for sialendoscopy were included in the study. Main outcome measures: The item generation process included a review of published data as well as interviews with patients. An expert panel then tested the content validity of the instrument and the construct validity was tested in 120 patients. Patients completed a self-administered CSOC questionnaire, a Short Form-36 and a Visual Analogue Scale. Feasibility, reliability, internal consistency, construct validity and responsiveness were assessed. Results: All the patients found the instrument understandable. Cronbach α coefficient was high (0.85). The time required to fill out was 5.7 and 4.5 minutes for pre and postsialendoscopy CSOC respectively. Cronbach α coefficient was very high for both pre and postsialendoscopy CSOC (0.90 and 0.94 respectively). The correlation with the SF-36 dimensions was negative and positive with the VAS. The mean score of CSOC was 28.6 and 7.98 for pre and postsialendoscopy Conclusions: The CSOC questionnaire is understandable, feasible, reliable and representative of quality of life in COS. Keywords: sialadenitis, QOL, questionnaire, validation, sialendoscopy