Objective: To determine the proportions of female participants in research studies selected to inform the development of national clinical guidelines and to assess these against the proportions of women affected by the conditions. Methods: We assessed 392 published articles, involving a total of 5.2 million participants, cited as references in five influential clinical guidelines addressing the use of antiarrhythmics, chronic fatigue, depression, diabetes, and colorectal cancer. For each article, we extracted the number of female participants to determine any discrepancies in the sex of participants and if the proportion of female participants as research subjects reflected the sex distribution of patients affected by the condition. Results: The overall and median percentages (per study) of females per guideline were: use of antiarrhythmics (35%, median 38%), chronic fatigue (70%, median 73%), colorectal cancer (67%, median 46%), depression (66%, median 66%), and diabetes (63%, median 50%). The baseline prevalence rates used for comparison purposes were (percentage female): antiarrhythmics (60% of patients 75 + years); chronic fatigue (66%), colorectal cancer (46%), depression (66%), and diabetes (46%). Conclusions: The colorectal cancer, depression, and chronic fatigue guidelines were based on research populations that accurately reflected the sex distribution of the condition in the general population. Women were slightly overrepresented in the research studies supporting the diabetes guidelines and were significantly underrepresented in the research studies supporting the guidelines on the use of antiarrhythmics. Guideline developers should be aware of and comment on the potential impact of sex. Where the evidence base is lacking, guideline developers should highlight this and, where necessary, limit their specific conclusions to populations on whom the research was performed.