A malignant lymphoma arising in the lung of a pediatric HIV-positive patient exhibited histologic and clinical features of low-grade B-cell lymphoma of mucosa-associated lymphoid tissue (MALT). Clinically, the neoplasm consisted of a 4-cm mass in the left-upper lobe of the lung of a 7-year-old girl. The lung mass was surgically resected. Monoclonal immunoglobulin heavy and light chain gene rearrangements were shown by Southern blot. Monoclonality of light chain expression was demonstrated by immunohistochemistry. Coexpression of Leu-22 (CD43) by the tumor cells supported the diagnosis of lymphoma. The remainder of the pulmonary parenchyma distal to the mass was associated with pulmonary lymphoid hyperplasia/lymphocytic interstitial pneumonitis, which may have been a predisposing factor. Gastric MALT lymphomas have recently been described in adult HIV-antibody-positive patients. Ours represents the first reported case of a pulmonary MALT lymphoma in a pediatric HIV-positive patient. In addition, at age 7, this is the youngest patient reported with a MALT lymphoma.