In general, the larger the tumor, the higher the incidence of malignant lesions. For SMT with a maximum diameter >3 cm, complete resection is currently recommended because of its high risk of malignancy. However, there is still a lack of clinical evidence regarding the treatment of SMT with a maximum diameter of ≤3 cm. The majority of SMTs with a small maximum diameter have benign features and rarely become malignant, and it has been suggested in the literature that patients with SMTs with a small maximum diameter can be followed up periodically. Notably, the incidence of smooth leiomyosarcoma is more common in esophageal SMT, and the incidence of malignant lesions is extremely low even in large tumors. Small bowel SMT usually has obvious clinical symptoms and is mostly malignant, and should be resected upon detection regardless of the size of the tumor, but there is still much controversy. However, rectal SMT with a maximum tumor diameter of <2 cm is mainly carcinoid tumors without risk of invasion and can be followed up regularly.