The aim of this study was to assess the level of understanding of the role of the tumor marker CA125 in ovarian cancer among doctors of different grades specializing in obstetrics and gynecology (O&G), medicine, surgery, and primary care (general practitioners [GPs]). The study involves a questionnaire-based survey. Two hundred and fifty-nine questionnaires were distributed. An overall response rate of 47.1% was achieved. All grades of doctors and all major specialties were represented. There was a significant difference in the level of self-reported CA125 ordering between the medical specialties, O&G being the most frequent users and primary care the least (P < 0.001), and between the grade of doctors, senior house officers/preregistration house officers and GPs less than consultants and middle grade doctors (P < 0.001). Electronic literature was the first source of advice for the majority of respondents (38.5%). The knowledge of false-positive causes for a raised CA125 was low in medicine, surgery, and primary care specialties, as was the awareness of the sensitivity and specificity of CA125 in epithelial and nonepithelial ovarian cancers. The role of CA125 in ovarian cancer is poorly understood, especially among doctors working outside O&G. Guidelines should be developed to aid clinicians from all specialties in the most appropriate application of CA125 in their practice. Substantial cost savings could be made by the introduction of clear protocol-driven ordering in an attempt to reduce the number of inappropriate tests performed.