Qualitative research has been increasingly recognized in recent years as having a distinctive and important contribution to make to health care research. It is capable of being used as a methodologically sufficient approach in its own right, as a precursor to quantitative studies, during or after trials to explain processes and outcomes, and as a means of enhancing the link between evidence and practice. However, qualitative research has been little used as an evidence resource for systematic reviews. We argue that formal synthesis of both qualitative and quantitative forms of research is essential, and we discuss some of the problems that need to be overcome in carrying out such syntheses. These include methodological prejudice, problems in searching for qualitative evidence, and issues in synthesizing qualitative data. We call for progress to be made on the science and methods of including qualitative research in the evidence base of medicine.