Whether a 0.4 cm nodule in the lung is serious depends mainly on the nature of the nodule. Nodules smaller than 5mm are mostly benign nodules, which are chronic inflammatory changes with no clinical symptoms and do not require special treatment, and only need to be followed up once a year with high-resolution CT of the chest. If the lung nodule of 0.4 cm grows into a nodule of 6-8 mm or more within 6 months or 1 year, it means it is a growing nodule and needs to be given further chest enhancement CT examination. Especially, nodules with pleural traction sign, pleural depression sign, lobar shape, short burr need to be paid high attention to know whether early lung cancer may occur as soon as possible. If necessary, percutaneous lung puncture cell tissue biopsy should be given to assess whether early stage lung cancer has occurred. Once early lung cancer is formed, early surgical resection treatment by a thoracic surgeon needs to be performed as soon as possible to avoid delay in treatment. In conclusion, a lung nodule of 0.4 cm is not a serious status, but mainly to understand the dynamic growth and whether the nodule has changed in nature. In particular, some solid nodules are prone to early carcinogenesis and require high attention.