We report a retrospective study of the pathological features in 69 primary pulmonary non-Hodgkin's lymphomas which have previously been clinically reviewed. The tumours consisted of 61 (88%) low-grade and eight (12%) high-grade malignant lymphomas. Fifty-four of the low-grade malignant lymphomas were MALT lymphomas. Lymphoepithelial lesions were observed in bronchial, bronchiolar and alveolar lining. All tumours were composed of nodules, forming a lymphangitic pattern at the periphery and a confluent central mass. Invasion of pleura and vessels was often seen but this without any consequence on survival. Granulomas were found in 20% of cases. Six of the eight high-grade tumours were centroblastic and another two were B-cell lymphomas of undetermined type. In four cases, associated areas of low-grade malignant lymphoma with lympho-epithelial lesions indicated a preexisting MALT lymphoma. Clinical data suggest that limited surgery or non-aggressive chemotherapy can provide long-term survival in patients with such slowly developing neoplasms. However, non-invasive diagnostic methods need to be developed.