Easton et al. BMC Public Health 2010, 10:459 http://www.biomedcentral.com/1471-2458/10/459 relation to different aspects of health and healthcare. This has been highlighted as an issue across the body of health literacy research [45]. Most of the papers in the review reported on studies conducted in the US. The findings may not be general- iseable to other countries. For example, in the UK, use of the National Health Service does not require patients to engage in complex funding-related paperwork to access health services, and the reduced employment prospects associated with low health literacy would not have such significance for access. Nevertheless, the National Health Service in the UK provides most of its health advice and support for management of long term conditions in written form and patients are expected to read and implement medication instructions and drug warning labels on medicines obtained on prescription and over the counter. Overall, the findings of associations between lower functional or health literacy and (a) poorer adherence to recommended/prescribed health care interventions and (b) poorer health are broadly similar for the ‘hidden population’ of people with lower health literacy as for people who may have language and/or obvious cogni- tive/communicative impairments in addition to health literacy difficulties [17-20]. In general, the stigma asso- ciated with low literacy skills and the coping mechan- isms implemented by those with low literacy are likely to lead to refusal of many individuals from the popula- tions of interest to participate in research. This may underestimate the prevalence or effects in some studies and particularly in the ‘hidden population’ we have focused on. Conclusions The review has identified some evidence of association between low functional or health literacy in the ‘hidden population’ of people with literacy problems, but a num- ber of important questions remain unanswered. Further research is necessary to be able to understand the diffi- culties faced by people within this ‘hidden population’ in accessing health care and in self-care activities and to identify the mediators and moderators in the relation- ship. Studying such a population presents many difficul- ties, not least in identification of individuals with functional or health literacy problems. It is important for healthcare staff to bear in mind that there is a ‘hid- den population’ of people with low health literacy who appear to experience similar barriers to health and health services as those whose health literacy problems may be more obvious. Page 9 of 10 Additional material Additional file 1: Inclusion criteria for papers. 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