A left adnexal cyst of 29mm*21mm is not serious if it is a benign lesion. If the cyst is ruptured, has a twisted tip or has become malignant, the condition is more serious. If the cyst is benign and the size of the cyst is 29mm*21mm, the size is not large, so the degree of the disease is not serious. In general, benign adnexal cysts with a diameter of less than five centimeters can be treated with medication as prescribed by the doctor, such as oral triamcinolone acetonide and other non-steroidal anti-estrogen drugs, to accelerate the formation of progesterone in the patient’s body and to promote the recovery of the disease. If the adnexal cyst ruptures or torsion occurs during the course of the disease, it will be more serious and may require surgery. If the adnexal cyst has become malignant, it will be more serious and need to be treated with surgery, radiotherapy and chemotherapy as soon as possible according to the doctor’s instructions. Otherwise, delayed and untimely treatment may jeopardize the patient’s life. Once diagnosed with a left adnexal cyst, it is recommended to consult a doctor in a timely manner and choose the appropriate treatment according to the degree of the patient’s condition.