Nonattendance to scheduled appointments in outpatient clinics is a frequent problem in ambulatory medicine with an impact on health systems and patients' health. The characterization of nonattendance is fundamental for the design of appropriate strategies for its management.To identify causes of nonattendance of scheduled ambulatory medical appointments by adult patients.Case and two controls study nested in a prospective cohort. A telephone-administered questionnaire was applied within the first 72 hours to identify the causes of attendance, nonattendance, or cancellation in patients who had a scheduled appointment to which they had been present, absent, or cancelled.A total of 150 absences (cases), 176 attendances, and 147 cancellations (controls) in a prospective cohort of 160 146 scheduled appointments (2012/2013) were included. According to self-reports in telephone interviews, the most frequent causes of nonattendance were forgetting 44% (66), unexpected competing events 15.3% (23), illness or unwellness 12% (18), work-related inconvenience 5.3% (8), transport-related difficulties 4.7% (4), and cause that motivated appointment scheduling already resolved 4.7% (4).The main cause of nonattendance is forgetting the scheduled appointment, but there is a proportion of different causes that do not respond to reminders but could respond to different strategies.