Most patients have more than one disease, and these diseases are able to affect one another. In modern medicine, the etiology and pathophysiology of diseases are well known in detail. However, inter-disease relationships are still mysterious. Physicians’ knowledge and experience have great importance in such a multi-criteria case. Because medical doctors in internal medicine clinics deal with large numbers of patients with multiple diseases, they have quite a complex approach in treating illness. In this context, exposing the cause-and-effect relationships among diseases frequently seen in internal medicine will contribute to physicians’ ability to blend profound theoretical knowledge with experiential results. Therefore, this study presents a fuzzy DEMATEL (Decision-Making Trial-and-Evaluation Laboratory) method to assess the most common diseases in internal medicine outpatient clinics. The DEMATEL method allows one to identify and analyze significant diseases in internal medicine by considering the cause-and-effect relationship diagram. Likewise, fuzzy sets in DEMATEL overcome the uncertainty in making decisions about disease relationships and internal medicine experts’ judgments. When investigating the results, we have found dyspepsia, hyperlipidemia, and anemia to be crucial in terms of causes. When evaluating the effects, the most notable diseases are understood to be renal failure, malignancy, and hepatitis. The results indicate that in the presented study, we could successfully apply these methods to reveal the cause–effect of diseases. The results of this study will contribute to understanding the complex multi-criteria relationship among internal diseases using internists’ opinions.

大多数患者患有一种以上的疾病，这些疾病能够相互影响。在现代医学中，疾病的病因学和病理生理学是众所周知的。但是，疾病间的关系仍然是神秘的。在这种多标准案例中，医师的知识和经验非常重要。由于内科诊所的医生会处理大量患有多种疾病的患者，因此他们在治疗疾病方面有相当复杂的方法。在这种情况下，揭示内科常见疾病之间的因果关系将有助于医师将深刻的理论知识与实验结果相结合的能力。因此，这项研究提出了一种模糊的DEMATEL（决策试验与评估实验室）方法来评估内科门诊中最常见的疾病。DEMATEL方法允许通过考虑因果关系图来识别和分析内部医学中的重大疾病。同样，DEMATEL中的模糊集克服了在确定疾病关系和内科专家判断时的不确定性。在调查结果时，我们发现消化不良，高脂血症和贫血在病因方面至关重要。在评估效果时，最值得注意的疾病应理解为肾衰竭，恶性肿瘤和肝炎。结果表明，在本研究中，我们可以成功地应用这些方法揭示疾病的因果关系。