Eleven patients with localized lymphoid nodules of the lung (LLN) were seen at the Mount Sinai Hospital from 1962-1981. The diagnosis of pseudolymphoma was made in six instances based on the following criteria: (1) solitary or multiple nodules discovered on chest roentgenograms composed of cytologically benign lymphoid cells (small lymphocytes); (2) polymorphic character of the infiltrate, including plasma cells, histiocytes and monocytes; and (3) presence of germinal centers in the lesion. Five lesions were classified as lymphomas and exhibited: (1) solitary or multiple nodules composed of atypical lymphoid cells; (2) absence of germinal centers; (3) lack of mediastinal lymph node involvement. Bronchial and/or pleural infiltration by lymphoid cells was present in lymphomas as well as in pseudolymphomas. Five lesions were studied with immunofluorescent techniques for the presence of intracytoplasmic immunoglobulins and in one pseudolymphoma, lymphocyte marker studies were performed. The procedures were not useful in separating benign from malignant lesions. All patients underwent surgery; three with lymphoma and one with pseudolymphoma received adjuvant chemotherapy. Patients were followed post surgically for up to 13 years. None of the six patients with pseudolymphoma died as a result of their lesions but two had either recurrences or developed extrapulmonary lymphoid lesions. All five lymphoma patients did well. Only one died while on chemotherapy with invasive pulmonary aspergillosis but no tumor. One-hundred and sixty-seven reported cases from the literature are analyzed. Pulmonary pseudolymphomas do not necessarily follow a benign course and malignant lymphomas limited to the lungs do not usually undergo progressive disease. Present pathologic criteria do not allow prediction of recurrence or progression of disease and are not acceptable for determining the advocacy of chemotherapy in patients with LLN.