Although there has been extensive research into the mechanisms involved in glomerular crescent formation, it is not yet fully understood how this change may cause renal function impairment. The aim of this study is to identify morphologic changes which may be responsible for this phenomenon. Thirty-eight renal biopsies showing glomerulonephritis with extracapillary proliferation (20 vasculitis-related, 6 idiopathic, 9 due to immune-complex deposition and 3 superimposed on diabetic nephropathy) were considered, and 146 glomeruli in which both crescents and the urinary pole were found at the same time, were studied. The involvement of the urinary pole by cellular crescents was observed in 93.1 and 100% of the glomeruli with segmental or circumferential crescents, respectively. A tridimensional study, for the evaluation of the glomeruli as a whole, was performed on 8 biopsies by means of the step-section technique and disclosed the involvement of the urinary space and a close contact between crescent and tubular cells in all 54 investigated glomeruli. The reported features do not seem to be related to the type of cells which formed the crescent. Indeed, as shown by immunohistochemical study on 10 cases with anti-cytokeratin and anti-CD68 antisera, the crescent localization at the urinary pole had no correlation with the prevalence of epithelial or macrophagic cells. These findings suggest that crescents, due to epithelial proliferation or macrophage clustering, tend to localize at the urinary pole and thus come into close contact with cells of the proximal convoluted tubule: the formation of a sort of plug or a ‘glomerular stone’ could well explain the block in the urine flow and the consequent impairment of renal function in the acute phase of the disease, even in those cases where crescents are segmental.