This study investigated the user activity in a text messaging intervention to assist mindfulness practice in patients with symptoms of depression. Participants (N1⁄421) allocated to the intervention group of a pilot RCT received an introduction to mindfulness exercises during hospital treatment, and text messages to assist mindfulness practice during follow-up. Results showed that text messaging activity decreased over the course of the intervention. Participant characteristics showed moderate associations with text messaging activity. User activity was moderately associated with clinical outcome. We highlight the careful integration of mobile interventions in mental health services in order to maximize patient en- gagement and outcome.