Patients with pseudolymphoma, well-differentiated lymphocytic lymphoma, and lymphocytic interstitial pneumonitis were studied. Diagnoses were confirmed by recently developed immunochemical staining techniques. Radiographic findings in pseudolymphoma were different from those in well-differentiated lymphocytic lymphoma, and clinical findings in lymphocytic interstitial pneumonitis were different from those in the other two types of lesions. The interstitial infiltrate of lymphocytic interstitial pneumonitis had two patterns: basilar with an alveolar component and diffuse with associated honeycombing. Pseudolymphoma and lymphocytic interstitial pneumonitis are being polyclonal inflammatory processes without malignant potential. Well-differentiated lymphocytic lymphoma, a monoclonal neoplasm, often has an indolent course.