Background: Meta-analyses and systematic reviews of observational studies are increasingly used to synthesize epidemiological results for decisions in public health, research and clinical practice. The interpretation of a summary effect estimate from observational data needs to consider potential sources of bias in the original study and the diversity of study populations. The association between selenium and cancer has been widely debated in recent years and stimulated a large randomized controlled trial on the use of selenium supplements for prostate cancer prevention in US American men. However, results will not be available before 2011. Generalizability to other populations - e.g. women and non- US residents - is open to question, as differential influences of sex-specific hormones and and baseline selenium exposure on health are discussed. Objectives: To investigate and highlight the diversity of populations represented in prospective observational studies on selenium exposure and cancer risk and the implications of this diversity on practice and research. Methods: We conducted a Cochrane review on ‘‘Selenium for cancer prevention’’. In this paper, all eligible cohort and nested case-control studies investigating the association between selenium exposure and cancer risk are included. Results: The search strategy identified 66 eligible papers reporting data from 44 different studies on more than 20 different cancer types. The investigations included about 737,00 individuals (about 45% women) from 37 cohorts in Asia, Australia, Europa and the US. The majority included mixed gender populations, but did not report gender disaggregated data. We will describe the representation of both genders, and the diversity of baseline selenium exposure levels in different global regions in the included studies. Conclusions: The studies reviewed originated predominantly from Europe and the US; no study from Africa or South America could be identified. The lack of gender disaggregated data inhibits the understanding of a possible sex/gender-specific association between selenium exposure and cancer risk. We will discuss the representation of regional diversity and gender in the included studies and the possibilities and limitations the available data provide for a global perspective on selenium-related health effects in public health and research.