A systematic review of the literature to identify risk factors associated with Clostridium difficile infection was conducted. Two main outcomes were considered: C. difficile diarrhoea and C. difficile carriage. A qualitative assessment, based on a set of defined and consistently applied criteria, appeared to be the best approach for risk factors other than antibiotic use, as an approach based on meta-analysis would have utilized only the information provided by a minority of the studies. Risk factors for which there was evidence suggestive or consistent with an association with C. difficile diarrhoea were: increasing age (excluding infancy), severity of underlying diseases, non-surgical gastrointestinal procedures, presence of a nasogastric tube, anti-ulcer medications, stay on ITU, duration of hospital stay, duration of antibiotic course, administration of multiple antibiotics. For malignant haematological disorders there was evidence of an association only with C. difficile carriage, but there were no suitable studies to explore a possible association of this risk factor with symptomatic infection. Antibiotic use lent itself to quantitative assessment with meta-analysis using logistic regression. Exposure to an antibiotic was shown to be statistically significantly associated with both C. difficile diarrhoea and C. difficile carriage. The meta-analysis approach enabled the ranking of individual antibiotics in relation to the risk of C. difficile infection, though the 95% confidence intervals were often wide and overlapping. Antibiotics associated with a lower risk of C. difficile diarrhoea should be considered, especially when attempting to control a C. difficile outbreak or when prescribing for a patient with other C. difficile risk factors. This systematic review of the literature enabled the identification of features it would be desirable to consider in future epidemiological studies.