IMPORTANCE Little is known about patients' experiences of advanced basal cell carcinoma (aBCC) and basal cell carcinoma nevus syndrome (BCCNS), a rare genetic disorder that greatly increases the number of BCCs. OBJECTIVE To develop a questionnaire to measure patient-reported outcomes (PROs) in these populations. DESIGN, SETTING, AND PARTICIPANTS Concept elicitation interviews were conducted with patients with aBCC and BCCNS from 5 US clinical sites and the BCCNS Life Support Network and 4 physicians. The PRO questionnaires were drafted based on results from a literature review and findings from these interviews. Questionnaires were finalized after cognitive debriefing interviews were conducted with patients. Concept elicitation interviews were conducted with 30 patients (14 with aBCC, 16 with BCCNS) and 4 physicians (2 dermatologists, 1 Mohs surgeon, and 1 oncologist) in the United States. A subset of 10 of these patients (5 with aBCC, 5 with BCCNS) took part in cognitive debriefing interviews. MAIN OUTCOMES AND MEASURES Development of 2 questionnaires to allow clinicians to assess the emotional, social, and physical impacts of the disease on patients with aBCC and BCCNS. RESULTS Most concept elicitation interview patients were male (63%) and white (93%); their mean age was 57 years. There were impacts on emotional, social, and physical functioning in both conditions. Patients were unable to do many activities and avoided other activities. Seventy-nine percent of patients with aBCC and all patients with BCCNS reported scarring. Physician interviews revealed similar findings. During cognitive debriefing interviews, the questionnaires were found to be relevant, clear, and comprehensive. CONCLUSIONS AND RELEVANCE Advanced BCC and BCCNS affect patients in unique and substantial ways. These PRO questionnaires were developed with patient and clinician input and measure the key areas that have an impact on patients with these conditions.