Non-attendance at outpatient clinics is a common problem, resulting in inefficiency and wasted resources. To establish the reasons why patients failed to attend their outpatient clinic appointment and to assess what proportion of these failed attendances were potentially preventable, we conducted a 13-month prospective postal survey of clinic non-attenders to our hospital. Of 5248 appointments made during the study, 521 were not kept (9.9%). Of these, 224 patients replied to the questionnaire, a response rate of 43%, with new patients more likely to not attend than old patients (odds ratio 2.7, P < 0.001). From the replies, 27.3% of non-attendances could be described as 'clerical error' with a further 17.9% due to patients failing to remember their appointment. Based on the replies received, a better organized outpatient booking system and a simple postal reminder could potentially prevent at least 40% of non-attendances.