Chikio Hayashi has conducted a variety of medical research throughout the wholecourse of his life and made many remarkable achievements in the field of medicine. Dur-ing his long carrier, he always noticed: “In clinical medicine we, the researchers, have tokeep in mind that we are not merely dealing with numbers or things, but we are dealingwith human life which is a part of society. They are people, patients who suffer fromdiseases. Therefore, in statistical analysis of clinical data, researchers have to considerthat these data were taken from human life.” In this paper, we have tried to followDr. Hayashi’s footprints and focus on his initial interest in clinical medical researchin the early 1950’s to the establishment of his methodology, “data science” in the late20th century. First, we reviewed the times he engaged in medical studies along withother specialists of medicine. Secondly, we reviewed the times he rebuilt the concepts of“risk factors” which has been used traditionally in clinical medicine from the patients’point of view. Lastly, we shed light on how Dr. Hayashi’s methodology, “data science”is suitable for dealing with complex problems in clinical medicine.