OBJECTIVES: This study evaluated the qualitative equivalence between paper and electronic diary (eDiary) versions of the following 6 patient-reported outcome (PRO) instruments used in endometriosis studies: Dysmenorrhea (DYS), Non-menstrual Pelvic Pain (NMPP), Dyspareunia, Uterine Bleeding, Numeric Rating Scale (NRS), and Menstrual Period. Study medication and Analgesic Medication questions were also assessed for usability. Equivalence evaluation of these questionnaires in an eDiary was needed to document suitability of this mode of data collection for upcoming clinical trials. METHODS: A cross-sectional qualitative study was conducted involving cognitive and usability interviews with premenopausal women diagnosed with endometriosis recruited from 2 US sites. The 6 symptom questionnaires and 2 medication questionnaires were administered on an HTC HD2 eDiary and paper versions. Participants were randomized to order of mode completion to control for order effects.  Interviews were conducted in two rounds to allow for evaluation of issues between rounds. RESULTS: Mean age of the sample (N=10) was 31 years, (range 25-46), 70% were white; 50% were employed part-time; 50% had completed secondary school or some college, while 50% had completed a college degree. Participants found the training useful, device easy to use, considered the platforms and formats to be similar, and preferred the eDiary to paper. No usability issues were noted in the PRO or medication questions, although one participant suggested a larger device would be easier for some participants to read. Only one instance of a discrepancy in response between the formats occurred, and was due to perceived similarity between the adjacent responses Mild and Moderate, not due to the device.  No changes were made between rounds. CONCLUSIONS: The study showed excellent qualitative equivalence between paper and electronic versions of 6 PRO questionnaires.  This study also supported usability of the eDiary with an endometriosis population which expressed a strong preference for the electronic version.