Thrombospondin is a multifunctional 450 kD glycoprotein which may be secreted into the extracellular matrix by a wide variety of cells. Occasional foci of immunoreactive thrombospondin have previously been demonstrated within normal human glomeruli. A specific polyclonal antibody directed against thrombospondin 1 was used to examine the distribution of this regulatory glycoprotein in renal biopsies from patients with a variety of renal diseases, including rapidly progressive glomerulonephritis associated with circulating antibodies to neutrophils, active or quiescent systemic lupus erythematosus, and membranous nephropathy, together with normal renal tissue. The results demonstrated the marked up-regulation of thrombospondin expression in acutely inflamed renal tissue with strongly positive, predominantly extracellular staining of glomerular crescents, although cytoplasmic staining of epithelial cells was also seen, indicating that these cells may contribute to thrombospondin accumulation at these sites. Occasional segmental mesangial staining was seen in cases of active lupus and rapidly progressive glomerulonephritis, while some focal interstitial staining around peritubular capillaries was seen in all renal tissue examined. These results suggest that thrombospondin may play an important role in the regulation of cellular recruitment, proliferation, and function in crescentic glomerulonephritis.