Medical decision-making requires years of experience in order to develop an adequate level of competence to successfully engage in safe practice. While diagnostic and technical skills are essential, an awareness of the extent and limits of our own knowledge and skills is critical. The present study examines clinicians' subjective awareness in a diagnostic cardiac ultrasound task. Clinicians answered diagnostic and treatment related questions for a range of pathologies. Following these questions, clinicians indicated their level of confidence in their response. A comparison of response accuracy and confidence revealed that clinicians were generally overconfident in their responses. Critically, we observed that a clinician's overconfidence was negatively correlated with prior experience: clinicians that had more prior experience expressed less overconfidence in their performance such that some clinicians were in fact underconfident. We discuss the implications for training in medical education and decision-making.