The most common cause of lymph node enlargement in the neck in a 17 year old is a viral illness.
Large lymph nodes, which are greater than 2 cm in size and persist for weeks are less common. These can be seen with mononucleosis, some drug reactions and other more serious conditions. If the lymph nodes persist for 2 months and the test for mononucleosis is negative a biopsy
should be done.