The DNA and nuclear antigens of Epstein-Barr virus (EBV) have been detected in specimens of tissue of non-Hodgkin lymphoma and lymphocytic interstitial pneumonitis from patients with acquired immunodeficiency syndrome. To determine whether there is serologic evidence of an active EBV infection in these disorders, we conducted a case-control study. The case patients were 10 children with acquired immunodeficiency syndrome and EBV genome-positive pneumonitis or lymphoma. We randomly selected one or, if available, two matched control patients with human immunodeficiency virus infection for each index patient and compared their EBV serologic profiles with those of the index case patient at the time of the biopsy. Ten case patients and 13 matched control patients were enrolled. All 10 case patients (100%) compared with 2 (15%) of 13 matched control patients had serologic evidence of either a primary or a reactivated EBV infection at the time the index patient had a biopsy performed (p less than 0.001). Therefore we found serologic and virologic evidence that EBV is etiologically related to EBV-associated lymphocytic interstitial pneumonitis and non-Hodgkin lymphoma in children with acquired immunodeficiency syndrome.