A 37-year-old white man had untreated lymphomatoid papulosis for 12 years before a submandibular T cell immunoblastic lymphoma developed. A genetic abnormality, composed of extra chromosomal material attached to the short arm of chromosome 9, was detected in the lymphoma tissue but not in the skin. The lymphomatoid papulosis skin lesions did not manifest clonal T cell receptor gene rearrangements, but the submandibular lymphoma tissue was clonal and of T cell lineage. The patient's lymphoma responded well to combination chemotherapy, but the lymphomatoid papulosis remains active.