BACKGROUND: The information given to people considering taking part in a trial needs to be easy to understand if those people are to become, and then remain, trial participants. However, there is a tension between providing comprehensive information and providing information that is comprehensible. User-testing is one method of developing better participant information, and there is evidence that user-tested information is better at informing participants about key issues relating to trials. However, it is not clear if user-testing also leads to changes in the rates of recruitment in trials, compared to standard trial information. As part of a programme of research, we embedded 'studies within a trial' (SWATs) across multiple ongoing trials to see if user-tested materials led to better rates of recruitment. METHODS: Seven 'host' trials included a SWAT evaluation and randomised their participants to receive routine information sheets generated by the research teams, or information sheets optimised through user-testing. We collected data on trial recruitment and analysed the results across these trials using random effects meta-analysis, with the primary outcome defined as the proportion of participants randomised in a host trial following an invitation to take part. RESULTS: Six SWATs (n=27,805) provided data on recruitment. Optimised participant information sheets likely result in little or no difference in recruitment rates (7.2% versus 6.8%, pooled odds ratio = 1.03, 95% CI 0.90 to 1.19, p-value = 0.63, I2 = 0%). CONCLUSIONS: Participant information sheets developed through user testing did not improve recruitment rates. The programme of work showed that co-ordinated testing of recruitment strategies using SWATs is feasible and can provide both definitive and timely evidence on the effectiveness of recruitment strategies. TRIAL REGISTRATION: Healthlines Depression (ISRCTN14172341) Healthlines CVD (ISRCTN27508731) CASPER (ISRCTN02202951) ISDR (ISRCTN87561257) ECLS (NCT01925625) REFORM (ISRCTN68240461) HeLP Diabetes (ISRCTN02123133).