evidence: To investigate the incidence of toxoplasmosis in patients with lymphoid hyperplasia of the head and neck. In this study, 53 patients (32 males, 21 females; mean age 11.2+/-4.3 years, range 5 to 22) were investigated. There were hypertrophic tonsillitis in 13 patients, hypertrophic tonsillitis and adenoid hyperplasia in 22 patients, adenoid hyperplasia in 10 patients, and lymphadenopathy of the neck in eight patients. In venous blood samples, IgM and IgG antibodies for Toxoplasma gondii were investigated with the enzyme-linked immunoabsorbent assay (Axsym, Abbott). Positive IgG was determined in 23 patients (48%) with hypertrophic tonsillitis (n=6), hypertrophic tonsillitis and adenoid hyperplasia (n=10), adenoid hyperplasia (n=3), and lymphadenopathy of the neck (n=4). Positive IgM was determined in 5 patients (9.4%) with hypertrophic tonsillitis (n=1), hypertrophic tonsillitis and adenoid hyperplasia (n=1), and lymphadenopathy of the neck (n=3). Toxoplasmosis, which is known to cause lymphadenopathy, may be a reason for lymphadenopathy of the neck. Toxoplasmosis should be considered in the differential diagnosis of untreated patients with tonsillitis, adenoid hyperplasia, and chronic neck lymphadenopathy.