Background
Recruiting the required number of participants is vital to the success of clinical research and yet many studies fail to achieve their expected recruitment rate. Increasing research participation is a key agenda within the NHS and elsewhere, but the optimal methods of improving recruitment to clinical research remain elusive. The aim of this study was to identify the factors that researchers perceive as influential in the recruitment of participants to clinically focused research.

Methods
Semi-structured interviews were conducted with 11 individuals from three clinical research teams based in London. Sampling was a combination of convenience and purposive. The interviews were audio recorded, transcribed verbatim and analysed using the framework method to identify key themes.

Results
Four themes were identified as influential to recruitment: infrastructure, nature of the research, recruiter characteristics and participant characteristics. The main reason individuals participate in clinical research was believed to be altruism, while logistical issues were considered important for those who declined. Suggestions to improve recruitment included reducing participant burden, providing support for individuals who do not speak English, and forming collaborations with primary care to improve the identification of, and access to, potentially eligible participants.

Conclusions
Recruiting the target number of research participants was perceived as difficult, especially for clinical trials. New and diverse strategies to ensure that all potentially eligible patients are invited to participate may be beneficial and require further exploration in different settings. Establishing integrated clinical and academic teams with shared responsibilities for recruitment may also facilitate this process. Language barriers and long journey times were considered negative influences to recruitment; although more prominent, these issues are not unique to London and are likely to be important influences in other locations.