Despite the availability of different classifications for rapidly progressive glomerulonephritis (RPGN), patients with "idiopathic crescentic GN" have not been yet inserted as a precisely defined subgroup, pointing to their probable heterogenicity. Trying to better define their characteristic, we retrospectively analyzed the clinical, histological and immunopathological features of 41 patients diagnostically labelled "idiopathic RPGN" because they had no evidence of systemic disease (including systemic vasculitis), no anti-GBM mediated glomerulonephritis and no clearly defined primary glomerulopathy. Starting by a thorough morphological review, 2 subgroups were defined: group I (25 patients) with variable degrees of intraglomerular necrosis, and group II (16 patients) with no intracapillary necrotizing lesions. Group I showed no or minimal endocapillary proliferation, intense interstitial infiltrates with periglomerular localization, frequent ruptures of Bowman's capsule and mild degree of glomerular and/or interstitial sclerosis. 16 patients in this group (64%) had irregular deposits of complement C3 at immunofluorescence while the remaining 9 (36%) had no immune deposits. Clinically they had no previous history of preceding urinary abnormalities, had a mean of 1.8 g/day proteinuria and a positivity for ANCA in 92% (12/13). In group II there was frequently marked mesangial proliferation, scarce interstitial infiltrates, no ruptures of Bowman's capsule and marked degrees of glomerulosclerosis and interstitial fibrosis. All patients in this group had clearly defined immune deposits of C3 and/or IgG. Clinically 50% of these patients had a history of recurrent microhematuria and/or proteinuria, a mean of 4.5 g/day proteinuria and negativity for ANCA in all 8 patients tested.(ABSTRACT TRUNCATED AT 250 WORDS)