Five cases of a distinctive type of undifferentiated large cell thymic carcinoma accompanied by an inflammatory reaction having morphologic features closely resembling those of Castleman disease (CD) of the hyaline vascular type (HVCD) are reported. The tumors occurred in 3 men and 2 women with a median age of 53 years. Three patients were asymptomatic and the tumors were found incidentally; 1 patient presented with fatigue, weight loss, dyspnea, and chest pain, and another with acanthosis nigricans. The tumors were characterized by the admixture of two components: a neoplasm of cytokeratin (+)/CD5(-) undifferentiated large tumor cells and an inflammatory reaction resembling the late stage of HVCD. Some of the cases were associated with a remarkably indolent clinical course, especially when considering their high-grade morphology. Three patients were alive without disease at 1, 10, and 22 years. One patient was alive with persistent disease at 1 year. One patient's clinical course was unique in that the primary thymic tumor was found 16 years after a metastasis had been detected in two distant lymph nodes. The two possible explanations for the coexistence here described are as follows: 1) a CD-like reaction to the tumor as the morphologic manifestation of a host immune response; and 2) a malignancy engrafted upon preexisting HVCD of the thymus, in a manner analogous to that operating in the reported cases of tumors of dendritic/reticulum cells complicating HVCD at other sites. The first hypothesis is favored, with the added suggestion that the CD-like reaction may bear a relationship to the peculiarly indolent behavior that these tumors exhibit.