To test the data quality, scaling assumptions and scoring algorithms underlying the Endometriosis Health Profile-30 (EHP-30) questionnaire: a questionnaire developed to measure the health-related quality of life (HRQoL) of women with endometriosis. METHODS: A cross-sectional postal survey to 727 women with surgically confirmed endometriosis recruited from an existing genetic linkage study (OXEGENE), The National Endometriosis Society (NES), UK and the outpatient gynaecology clinics of the Women’s Centre, John Radcliffe Hospital, Oxford. Tests of data quality included secondary factor analysis, internal reliability consistency, descriptive statistics of the data, missing data levels, floor and ceiling effects and corrected item to total correlation scores. RESULTS: Six hundred and ten women (83.9%) returned the questionnaire. Secondary factor analysis verified the domain structure of the EHP-30. All 11 dimensions were internally reliable with Cronbach’s scores ranging from 0.80 to 0.96. Missing response rates ranged from 0.2 to 1.3%, and all items were found to be most highly correlated with their own (corrected) scale. CONCLUSIONS: Results confirmed the factor structure, scoring and scaling assumptions of the questionnaire. The high rate of data completeness indicated that the EHP-30 was acceptable and understandable to the respondents, thereby verifying its suitability for measuring the HRQoL of women with endometriosis.