Human epididymis protein 4 (HE4) has received major attention as a potential tumor marker in epithelial ovarian cancer; however, evidence of significant overexpression of HE4 in several other human cancers is expanding. To assess the possible limitations or benefits of HE4 in a clinical setting, this review aims to systematically outline published results of HE4 tissue expression and serum HE4 levels in healthy individuals and patients with benign or malignant tumors. Our findings suggest scientific basis for a potential diagnostic ability of HE4 in gynecologic cancer and lung cancer, and further research is needed regarding other cancers. Yet, it is important to recognize that other malignancies can cause increased HE4 levels. Furthermore, attention should be paid to the influence of age and renal function on HE4 serum levels in future studies as well as in the clinic for proper interpretation of serum HE4 test results. Cancer Epidemiol Biomarkers Prev; 23(11); 2285-95. ©2014 AACR.