It has been debated for almost 30 years whether Paget's disease of bone results from paramyxoviral infection of osteoclasts (OCs). Paramyxoviral-like nuclear inclusions are found in OCs from patients with Paget's disease, and measles virus (MV) or canine distemper virus (CDV) messenger RNA (mRNA) transcripts have been detected by in situ hybridization in bone cells from pagetic lesions. Furthermore, immunocytochemical studies have shown the presence of several paramyxoviral species in OCs from patients with Paget's disease. However, others have been unable to detect paramyxoviral transcripts in bone samples from patients with Paget's disease or marrow cultures from involved sites of patients with Paget's disease. Furthermore, no one has been able to isolate an infectious virus from pagetic bone samples or marrow cells from patients with Paget's disease, and a full-length viral gene has not been sequenced from pagetic samples. In this study, we have obtained the full-length sequence for the MV nucleocapsid (MVNP) gene in bone marrow from an involved site from a patient with Paget's disease and more than 700 base pairs (bps) of MVNP sequence in 3 other patients with Paget's disease. These sequences were undetectable in four normal marrow samples studied simultaneously. The sequences from the patients contained multiple mutations that differed from the Edmonston strain MVNP gene. These findings are consistent with the presence of a chronic MV infection in affected sites from these patients with Paget's disease.