In a two-year period 117 patients with diagnosed mononucleosis sundrome and 207 patients with diagnosed lymphadenopathy were serologically examined. Of these 57 patients were immunodeficient and 267 immunocompetent. Acute infection with Epstein-Barr virus was proved in 17.09% of the cases with mononucleosis syndrome, 13.04% with lymphadenopathy i.e. 8.77% of the immunodeficient and 15.73% of the immunocompetent patients. The significantly highest rate was recorded among schoolchildren and adolescents (from 7 to 27 years of age). The occurrence of other viral (adeno-and cytomegalo-) and non-viral infections (toxoplasma gondii, chlamydia) in these patients has also been analyzed.