BACKGROUND: Patient-based symptom assessments are necessary to evaluate the effectiveness of medical treatments for gastroparesis. AIM: To summarize the development and measurement qualities of the Gastroparesis Cardinal Symptom Index (GCSI), a new measure of gastroparesis-related symptoms. METHODS: The GCSI was based on reviews of the medical literature, clinician interviews and patient focus groups. The measurement qualities (i.e. reliability, validity) of the GCSI were examined in 169 gastroparesis patients. Patients were recruited from seven clinical centres in the USA to participate in this observational study. Patients completed the GCSI, SF-36 Health Survey and disability day questions at a baseline visit and again after 8 weeks. Clinicians independently rated the severity of the patients' symptoms, and both clinicians and patients rated the change in gastroparesis-related symptoms over the 8-week study. RESULTS: The GCSI consists of three sub-scales: post-prandial fullness/early satiety, nausea/vomiting and bloating. The internal consistency reliability was 0.84 and the test-re-test reliability was 0.76 for the GCSI total score. Significant relationships were observed between the clinician-assessed symptom severity and the GCSI total score, and significant associations were found between the GCSI scores and SF-36 physical and mental component summary scores and restricted activity and bed disability days. Patients with greater symptom severity, as rated by clinicians, reported greater symptom severity on the GCSI. The GCSI total scores were responsive to changes in overall gastroparesis symptoms as assessed by clinicians (P = 0.0002) and patients (P = 0.002). CONCLUSION: The findings of this study indicate that the GCSI is a reliable and valid instrument for measuring the symptom severity in patients with gastroparesis.