Endometriosis is an estrogen dependent disease characterized by the growth of endometrial epithelium and stromal cells outside the uterine cavity. Lack of a clinical test results in a diagnostic delay of between 6-12 years. Recent studies suggest that miRNAs may be useful diagnostic tools; however, results remain equivocal. Use of different reference miRNA and definitions of control groups are factors postulated to contribute to the inconsistent findings in the literature. Serum samples were collected from women (n=53) undergoing laparoscopic surgery. Reference RNAs and symptomatic vs asymptomatic control groups were studied. Comparisons were made between cases and controls, controls and treated vs non-treated cases, and controls, endometriosis and adenomyosis. Data were compared by Mann Whitney U tests and Kruskal Wallis tests. A p value  0.05 was considered statistically significant. Our major finding was that reference RNA selection and categorization of patients influenced results. When using the most appropriate reference and comparing to asymptomatic controls, miR-9 was upregulated and miR-451a was downregulated in women with endometriosis. When using the most appropriate reference and comparing to symptomatic controls, miR-9 and 141 were downregulated in women with endometriosis. miR-9 and 141 were also downregulated in women receiving treatment. When using the most appropriate reference and comparing to women with adenomyosis, miR-451a, 20a and 122 were downregulated in women with endometriosis. While miRNA is the newest and has most promise as a diagnostic biomarker, miRNA expression results are extremely sensitive to multiple experimental variables. Literature continues to approach miRNA research using different definitions of control populations and different reference RNA. To replicate results and advance the search for miRNA as a diagnostic biomarker for endometriosis, a common methodology between labs will be necessary. / Thesis / Master of Science (MSc) / Endometriosis affects 10% of women within their reproductive lifespan. Since the gold standard for diagnosing endometriosis is laparoscopy, a diagnostic biomarker for endometriosis is of utmost importance. Currently there is no universally acceptable biomarker and circulating levels of miRNA appear to be promising but results have yet to be replicated. Replication of results may be dependent on reference RNA and definitions of control groups.