Whether or not a 3mm pulmonary nodule is dangerous depends on the nature of the nodule. A malignant nodule is dangerous and manifests itself as a nodule with unclear or blurred borders; clear and smooth borders are mostly indicative of a benign nodule. Primary tumors of the lungs, tuberculosis infections, metastases to the lungs, misshapen tumors, inflammatory pseudotumors, and adenomatous changes can cause the development of lung nodules. 3mm lung nodules are dangerous if they are malignant nodules, and the common features of malignant nodules include unclear borders, fuzzy edges lobular sign, burr sign, pleural pulling sign, vascular cluster sign, vacuolar sign, and ground glass components. On the other hand, benign nodules generally have uniform density, clear borders, and may be accompanied by calcification. Patients with 3mm pulmonary nodules should go to the hospital in time to improve the examination, determine the benign and malignant nature of the nodule as early as possible, and be treated under the guidance of physicians.