It is often said that “the eyes are the windows to the soul”. This refers to the fact that the eyes can reflect the inner world of a person. However, people may not know that the eyes, as an important organ of the body, also play a pivotal role in monitoring the health status of the body. Many systemic diseases have characteristic manifestations in the eyes, and eye examinations help in the diagnosis and prognosis of many systemic diseases. The fundus of the eye is the only part of the body where deep small arteries and veins can be seen, as well as the only part where the optic nerve can be observed. Many systemic vascular diseases, such as diabetes mellitus, hypertension, and renal disease, produce changes in the fundus vessels, causing edema, exudation, and hemorrhage, and are clinically classified into various types and stages according to the degree of vascular lesions. Therefore, the fundus of the eye also becomes a window to observe the systemic pathology. Ms. Wu has been suffering from diabetes for more than 10 years, and her vision in both eyes has been decreasing year by year, especially in the past year, and sometimes her near vision is good and bad at times. Upon examination, she had a more severe cataract and had scattered microangiomas and hemorrhages on the retina, with punctate exudation in the visually sensitive macula. At the same time, due to poor blood glucose control, myopia deepened as the lens absorbed water and became convex with the rise of blood glucose; vision could be restored when blood glucose was lowered. These are all ocular complications in patients with mid- to late-stage diabetes. In addition, diabetes can cause chronic uveitis, neovascular glaucoma and ocular motility disorders. Depending on the extent of the lesions, diabetic fundus lesions can be divided into two types: simple and proliferative, each of which is divided into three stages. In the advanced stage of the disease, extensive retinal ischemia, neovascularization, vitreous hemorrhage, fibrosis and retinal detachment may occur, resulting in vision loss. To prevent this from happening, diabetic patients must undergo annual fundus examinations, through which doctors can understand the progress of the lesions as early as possible, determine the prognosis, and give the necessary medication combined with laser treatment or even surgery. This is true for diabetes, as well as vascular lesions such as atherosclerosis, hypertension, and nephropathy, and regular fundus examinations are especially important for patients. The fundus is like a watchtower, providing clinicians with timely early warning signals that facilitate the development of treatment plans and the assessment of efficacy. A few weeks ago, I treated a young man from Sichuan. He had recurrent fundus hemorrhages in both eyes and had been treated several times at the local hospital with poor results, and had lost his job opportunities due to low vision. On examination, the patient was found to have vitreous hemorrhage and fibrosis in one eye and patchy hemorrhage around the retinal vein in the opposite eye with vascular occlusion and neovascularization. The initial diagnosis was retinal perivasculitis in both eyes. Since the condition is most often seen in young men with tuberculosis, it was determined that this patient may have a history of tuberculosis. After taking a medical history, we learned that the patient had indeed had tuberculosis more than a year ago. After treatment with a combination of Chinese and Western medicine, the patient’s condition was controlled and his vision was restored. The characteristic ocular manifestations also facilitate the diagnosis of some difficult diseases. A few years ago, I visited the cardiovascular medicine department for a fundus consultation with a young man from a rural area who came to Shanghai to work. Before admission, he had fever of unknown origin, shortness of breath, and chest tightness. The patient also had symptoms of blurred vision. Fundus examination revealed exudative changes in the central retina with a morphology that approximated age-related macular degeneration, which is extremely rare in young patients. The patient was from a rural area and had a history of cat and dog contact, and toxoplasmosis, a rare parasitic disease, was suspected at that time. The results of the antibody test confirmed this judgment. The internal medicine department then administered the right medication and the patient’s condition quickly improved.