Primary pulmonary hypertension (PPH) and Castleman's disease (CD) are rare conditions infrequently encountered in clinical practice. In this paper, two patients diagnosed with both of these diseases are reported. The authors speculate that rather than being a chance occurrence, these conditions are linked by a common angio-proliferative mechanism. Therefore, an association between infection with the human herpesvirus-8 and the diseases of PPH and CD was sought. Evidence of human herpesvirus-8 infection was found in the lung tissue and, specifically, in the plexiform lesions from one of the patients.