Background: We did a ‘study within a trial’ (SWAT), evaluating the effectiveness of the inclusion of a pen with a postal questionnaire, compared to no pen being included, on the retention rate in a large orthopaedic trial. Methods: The SWAT was embedded in the KReBS trial. The primary outcome was the proportion of 12-month questionnaires returned. Secondary outcomes were the proportion of questionnaires completed and time to questionnaire return. Binary data were analysed using logistic regression and time to return using Cox proportional hazards regression. Odds ratios (OR) and hazard ratios (HR) are presented, with associated 95% confidence intervals and p-values. Results: In total, 2305 participants were randomised into the SWAT. In the pen group, 1020/1145 (89.1%) of participants returned a questionnaire, compared to 982/1147 (85.6%) in the no pen group. The absolute difference in questionnaire return rate was 3.5% (95% CI: 0.8% to 6.2%; p=0.01). There were statistically significant differences in questionnaire return rate (OR 1.36; 95% CI: 1.06 to 1.74; p=0.02), questionnaire completion rate (OR 1.40; 95% CI: 1.11 to 1.78; p<0.01) and time to questionnaire return (HR 1.17; 95% CI: 1.07 to 1.27; p<0.01) favouring the pen group. Conclusion: This SWAT adds to the growing evidence base for whether pens are effective as an incentive for retention, and indicates their potential effectiveness. Registration: KReBS trial registered on 20 February 2019, ID ISRCTN87127065; SWAT registered on 1 April 2019, ID SWAT92.