Erosive human/murine (hu/mu) SCID arthritis, caused by unilateral engrafting of human rheumatoid arthritis synovial membrane (RA-SM) in the knee joints of SCID mice, was monitored for up to 18 weeks by scintigraphic, radiological, morphological and immunohistochemical analyses.^99mTc-DPD scintigraphy and histology revealed secondary, oligoarticular spreading of arthritis to contralateral knees and hips, but not to forelimb joints. Also, there were no extraarticular manifestations. At 18 weeks, surviving human cells were found within the pannus, but not directly at the cartilage erosion front, where fibroblast-like cells and macrophages of murine origin predominated. The latter cells also predominated in secondarily affected joints, where no human cells were detectable. Preventive depletion of murine NK-cells by anti-asialo-GMI antibodies, to check the influence of NK cells independently of strain and MHC system, combined with application of autologous human PBMN cells, had virtually no effects on the disease process. The completeness of the SCID defect was not critical, i.e. T cells were completely absent in the organs examined, and the presence of a few B cells in the spleen did not correspond to particular disease features. The SCID defect itself had a clear impact, since, in the chronic phase, SCID.bg and RAG-2^−/−knockout mice developed less consistent pathological/scintigraphic signs of disease than SCID mice. Thus, unilaterally-induced hu/mu SCID arthritis is an oligoarticular disorder of the hindlimbs. Murine macrophages and fibroblast-like cells appear responsible for tissue destruction in engrafted and non-engrafted arthritic joints.