OBJECTIVE: Patient-reported symptom severity measures are important for evaluating the effectiveness of treatments for gastrointestinal diseases because they are the only measures in clinical evaluation that directly reflect the patient experience. The objective of this study is to describe the development and initial testing of a new patient self-report instrument, the Patient Assessment of Upper GastroIntestinal Disorders-Symptom Severity Index (PAGI-SYM) in subjects with gastroesophageal reflux disease (GERD) or dyspepsia. METHODS: Instrument content was based on an extensive review of the published literature and interviews with patients and clinicians. A sample of 448 persons with GERD (249) or dyspepsia (199) were identified and recruited from a large population survey. Using telephone interviews, subjects completed the PAGI-SYM, the SF-36, and a measure of patient-rated change in GI-related symptoms, the Overall Treatment Effect Scale (OTE). Two-week reproducibility was evaluated in 68 subjects. RESULTS:: The 37-item PAGI-SYM is comprised of 6 subscales: heartburn, reflux/regurgitation, nausea/vomiting, abdominal pain, bloating/early satiety, and other symptoms. Subscale internal consistency reliability was good (alpha=0.74 to 0.92) with the exception of the two-item subscale, other (alpha=0.45); total score alpha levels were excellent (0.95). Item correlations by subscale were in the predicted direction; all were significant (r=0.29 to 0.77; p<0.001). Correlations between PAGI-SYM total and subscale scores and SF-36 PCS and MCS scores were in the predicted direction; all were significant (r=-0.28 to -0.64; p<0.0001). CONCLUSIONS:: Results suggest the PAGI-SYM is internally consistent with evidence of content and construct validity. Further research on the instrument’s reproducibility, sensitivity to change and the definition of clinically-meaningful change in GERD and dyspepsia symptoms is needed before use in clinical studies.