Little is known about which patients miss appointments or why they do so. Using routinely collected data from four practices, we aimed to determine whether patients who missed appointments differed in terms of their age, sex, and deprivation scores from those who did not, and to examine differences between the practices with respect to missed appointments. The likelihood of someone missing at least one appointment was independently associated with being female, living in a deprived area, and being a young adult. Living in a deprived area was associated with a threefold increase in the likelihood of missing an appointment, and the extent of this association was the same across all four practices. Interventions aimed at reducing missed appointments need to be based upon these findings.