Using a case-control design, non-attenders and attenders at a periodontal clinic were sent postal questionnaires in order to compare the sociodemographic characteristics of non-attenders and attenders. Non-attenders were also asked why they failed to attend. The main reason given was that they had forgotten about their appointment. Administrative error was the second most frequent category of response explaining non-attendance. Differences in sociodemographic characteristics between attenders and non-attenders did not appear to explain non-attendance, apart from car and telephone ownership. Time between notice of an appointment and the way patients received their appointments was associated with non-attendance. Non-attenders viewed their problem as less serious than attenders.