Yang deficiency pattern is a representative syndrome differentiation. This article is a study on to which categories of modern diseases yang deficiency is assigned by reference to modern clinical papers and the meaning of yang deficiency interpreted with a perspective of Korean Medicine and a modern perspective. Yang deficiency, yang qi deficiency, lack of yang qi and yang qi debilitation are the words found in "Nei Ching" and yang qi can be interpreted as something to warm, drive and arouse. Zhangzhongjing considered recovery or loss of Yang as the key to life in "Shanghanlun". Danxi proposed "Yang being liable to hyperactivity, Yin being insufficient" and emphasized pathological ministerial fire of Yang exuberance rather than physiological ministerial fire of Yang deficiency. Zhangjingyue proposed "Yang not being in excess, Yin being often deficient" and understood growth and decline of yin qi are all led by yang qi and put emphasis on true yin in addition to yang qi. Diseases of yang deficiency pattern are related with decline of metabolic level, hypofunction of internal secretion, disorder of immune function, disorder of automatic nerve system, sympathetic nerve inhibition, metabolic disorder of microelements, increase of cGMP, change of microcirculation, low speed of blood stream, kidney malfunction. Diseases related with kidney are sterility, polycystic ovary syndrome, spinal stenosis, edema, renal failure, IgA nephropathy, erectile dysfunction, nephritis, prostatitis, benign prostatic hyperplasia, decrease of adrenal cortical hormone by nephrotic syndrome, myelodysplastic syndrome. Disease related with heart are heart failure, arrhythmia, cardiomyopathy, atherosclerosis heart disease, hypertension, hyperlipidemia, pulmonary heart disease. Diseases related with spleen are irritable bowel syndrome, ulcerative colitis. Diseases related with liver are hypothyroidism, liver cirrhosis ascites, hepatitis B, chronic hepatitis, hepatic diabetes. Diseases related with lung are allergic rhinitis, cough variant asthma, bronchial asthma, pulmonary emphysema. And diabetes mellitus, metabolic syndrome, aplastic anemia, headache, encephalatrophy, Alzheimer's disease are also related with yang deficiency.