Background: As healthcare services become progressively more stretched, there is increasing discussion of ways in which technological adjuncts may be used to deliver more cost-efficient services. Before widespread implementation, however, the use of these adjuncts requires proper scrutiny of their effects on psychological practice. Aims: This research examined the effectiveness of SMS reminders on client appointment attendance and dropout in a psychological treatment setting. It was predicted that the reminders would result in increased initial appointment attendance, increased total appointment attendance, and decreased client dropout. Method: A randomized controlled trial investigated the impact of SMS appointment reminders (two levels: present or absent) on client attendance (three levels: attended, rescheduled, or did not attend) and dropout (two levels: completed treatment or terminate early). Participants (N = 140) at an outpatient psychology clinic were randomly allocated to either receive an SMS appointment reminder one day before their scheduled appointment, or to receive no reminder. Results: No significant differences were found between the SMS and no SMS conditions in relation to appointment attendance. There were more client dropouts in the SMS compared to the no SMS condition. Conclusions: The SMS appointment reminders were not effective at increasing appointment attendance. The current research suggests that there is more to client non-attendance in psychological settings than the simple forgetting of appointments. Technological adjuncts may be useful in increasing the cost-efficiency of current services; however, this research highlights the importance of understanding the effects of technology before widespread implementation.