Lymph node pain under the arm is mostly seen in lymphadenitis, and some tumors and nodal diseases may also present lymph node pain, but it is relatively rare in clinic. 1. Lymph node pain under arm is most commonly seen in lymphadenitis, which is caused by inflammation in lymphatic drainage area, and patients mostly have the history of infection in skin area and so on, which may be accompanied by local redness and swelling, and elevated infection indexes in blood. 2. Most of the lymph nodes caused by tumor are painlessly enlarged, but there are a few people with pain in clinic, and these lymph nodes are tougher in texture, poor in mobility, or adhered to the surrounding tissues, which can be clearly diagnosed by combining with pathological examination. 3. Nodular disease is a kind of systemic disease that can involve multiple tissues, in which lymph nodes are one of the commonly involved parts, and a few patients may have pain in the affected area, such as lymph nodes under the arm. The diagnosis of the disease relies on pathologic examination and usually presents as non-caseating granulomas. In case of pain in the lymph nodes under the arm, it is recommended to visit the hospital for prompt clarification of the condition and corresponding treatment.