OBJECTIVES: To assess whether timing of SMS reminders improved postal questionnaire return rates from participants in a randomized controlled trial (RCT). STUDY DESIGN AND SETTING: A Study Within A Trial (SWAT) embedded in a multi-centre RCT evaluating three treatments for frozen shoulder. Participants who provided a mobile telephone number were randomized to either pre-notification SMS on the day of the questionnaire mail-out or post-notification SMS four days following questionnaire mail out for the 3-month follow-up. The primary outcome was the proportion of participants who returned a valid questionnaire. A systematic review was undertaken to identify other embedded trials to perform a meta-analysis. RESULTS: Of the 269 participants, 122/135 (90.4%) returned a valid questionnaire in the pre-notification arm and 119/134 (88.8%) in the post-notification arm (difference of -1.6%; 95% CI of difference: -8.9%, 5.7%). There was no difference in time to response (HR=1.04; 95% CI: 0.80 to 1.34) or need for additional reminders (OR=0.71; 95% CI: 0.43 to 1.17). Meta-analysis of two RCTs showed no difference in response rates between pre and post-notification reminders (OR=0.78 95% CI: 0.42 to 1.45). CONCLUSION: Timing of SMS reminders did not improve response rates, time to response or affect the need for additional reminders.