A web survey was conducted to examine the experiences with, and understanding of medical decision-making among adults in South Korea. The subjects for the study were 500 residents, 19 years of age or older living in Seoul, Gyeonggi-do, and Incheon. The average satisfaction with the current health-care system was found to be 3.08 on a 5-point Likert scale. The degree of trust in health institutes was 3.15 and the degree of trust in medical doctors was 3.24. More than 90% of the respondents want to receive an accurate diagnosis when they experience a serious disease, and 73.2% want to be informed from the doctor directly. With respect to medical decision-making, 41.2% of subjects prefer a decision-making model that involves the doctor, the patient, and a guardian (usually a family member); 36.2% of subjects prefer that the relevant decisions be made by the patient in consultation with the doctor. More than 60.0% of subjects answered that they would respect the preference of their parents in medical decision-making, while 21.0% responded that it would depend on the condition. The gender, age, occupation, and education level all had very little influence on subjects’ responses; gender did exhibit some influence on certain items. The results of this study indicate that while Korean adults accept the importance of their own judgements in medical decision-making, many of them want to engage their guardians in the actual decision-making process.