The pain in the inner side of the left arm may be caused by trauma, local inflammation, neurological diseases, and bone tumors or tumor-like lesions. 1. Trauma: due to trauma and other violent factors leading to injury of bone, muscle, tendon and other tissues in the inner side of the arm, the patient may have pain in the inner side of the left arm. 2. Local inflammation: due to long-term arm activities and infection and other factors lead to local inflammatory reaction of the inner side of the patient’s arm, the patient appeared in the inner side of the left arm pain, swelling, and increased skin temperature and other symptoms. 3. Neurological diseases: Inflammation of the radial nerve, ulnar nerve and other nerves innervating the upper limbs leads to pain and numbness in the inner side of the left arm. 4. Bone tumor or tumor-like lesions: such as benign tumors of bone, malignant tumors of bone, metastatic tumors, and bone cysts and other diseases, resulting in bone destruction, compression of peripheral nerves and other tissues, which has never caused pain in the inner side of the left arm. There are more reasons for the pain in the inner side of the left arm, it is recommended to consult a doctor in time, to clarify the cause of the disease with the help of the doctor, and to carry out targeted treatment or therapy.