The clinical presentation and surgical and pathological findings of 46 children with unilateral tonsillar enlargement (UTE; age range 2 to 13 years, mean age 6.5) who underwent tonsillectomy for biopsy purposes between 1975 and 1995 were compared with those of 7 children who received treatment for tonsillar lymphoma (TL; age range 2 to 9 years, mean age 4.8) during the same period. There was no history of rapid tonsillar enlargement in children in the UTE group, and only 20 (43%) were symptomatic. Symptoms included recurrent sore throats in 10 patients (22%), snoring in 5 (11%), nasal obstruction in 4 (9%), and dysphagia in 1 (2%). No children had systemic symptoms or significant cervical lymphadenopathy. In contrast, tonsillar enlargement was observed to occur within a 6-week period in all children with TL, and 6 (86%) children had symptoms at presentation that included dysphagia in 5 (71%), snoring in 3 (43%), night sweats in 2 (29%), and fever and rigors in 2 (29%). Cervical lymphadenopathy greater than 3 cm was present in 6 (86%) children, while 1 child (14%) had hepatosplenomegaly. There was no histopathologic evidence of neoplasia in the UTE group, and a true discrepancy in size between the two tonsils was confirmed in only 21 of 44 (48%) cases. All 7 patients in the TL group had non-Hodgkin's lymphoma. All received chemotherapy, with 5 of the 7 cured and 2 dying of disease. The data suggest that tonsillectomy should be performed for biopsy purposes in UTE where there is a history of progressive enlargement, significant upper aerodigestive tract symptoms, systemic symptoms, suspicious appearance of the tonsil, cervical lymphadenopathy, or hepatosplenomegaly. The diagnosis of TL should also be considered when UTE is present in an immunocompromised child or one with a previous malignancy, when acute tonsillitis is asymmetric and unresponsive to medical treatment, or when rapid bilateral tonsil enlargement occurs. Observation is appropriate management for other cases of UTE.