Sixty-four patients with lymphoid lesions involving the lung were separated into three groups. In 32 patients, the predominant lymphoid cell population consisted of small, mature-appearing round lymphocytes with or without plasmacytoid features. This group, designated small lymphocytic proliferation (SLP), represents a heterogeneous group of pulmonary lymphocytic lesions including small lymphocytic lymphoma, lymphocytic interstitial pneumonia, and lymphoid hyperplasia (pseudolymphoma). Thirteen S L P patients were identified as having small lymphocytic lymphoma on the basis of monoclonality, progressive disease in other sites, or both. This group was morphologically identical to the remainder of the S L P patients, except for a higher incidence of plasmacytoid features (P = 0.003) and a greater degree of mast cell infiltration (P