We retrospectively studied the prevalence, histologic features, clinical correlations, and long-term outcome of the intrarenal vascular lesions of lupus nephritis (LN) in a series of 169 renal biopsies performed between 1980 and 1994 in 132 patients with systemic lupus erythematosus. The most common vascular lesions were nonspecific sclerotic changes, found in 37% of the biopsies (24% if only the cases with moderate to severe changes are considered). The other common vascular lesions were "immunoglobulin microvascular casts," found in 24% of the biopsies. Vasculitis and thrombotic microangiopathy were rare lesions and were seen in only 4 (2.4%) and 1 (0.6%) cases, respectively. Isolated sclerotic vascular changes were present in biopsies from older patients with a longer duration of LN, compared with the group with no vascular lesions, and were associated with a significantly higher prevalence of hypertension. Overall, however, the long-term renal and patient survival of this group did not differ significantly from that of the patients without vascular changes. Immunoglobulin microvascular casts (IMCs) ("lupus vasculopathy") were characterized by the presence of immunoglobulin deposition within the glomerular capillaries and small arterioles. In the present study we extensively investigated the morphologic and immunologic features of this lesion. The lesions were notable for the absence of endothelial or parietal vascular lesions and of fibrin, platelets, and leukocytes, which indicates that thrombosis is not involved in the vascular obstruction. According to our data immunoglobulin precipitation in the microvasculature seems to play a central role in the pathogenesis of this lesion, which is why we propose the term "immunoglobulin microvascular casts." In general, IMCs were associated with the most severe and active forms of diffuse proliferative lupus nephritis (World Health Organization [WHO] class IV). However our data show that, in contrast to previous studies, the long-term outcome of patients with IMCs is not worse than that of other patients with class IV LN. It may even be somewhat better, suggesting that this type of lesion may reverse with immunosuppressive therapy. In addition, we did not find any association between the presence of IMCs and the lupus anticoagulant, IgG anticardiolipin antibodies, or extrarenal vascular manifestations. Concerning vasculitis and thrombotic microangiopathy, our results confirm that their occurrence is quite rare in-lupus nephritis. The outcome of our 4 patients with vasculitis was not particularly poor, which could be related to early and/or aggressive treatment. Taken as a whole, our data confirm that the presence of active and severe forms of diffuse proliferative LN (WHO class IV) carries a worse prognosis compared with the other forms of LN. In our study, and in agreement with previous reports (23), the long-term renal survival of patients with class IV LN was significantly worse than that of patients with other forms of LN, with a 10-year renal survival of 70% compared with 85%, respectively. However our data do not support the conclusions of some previous studies that the presence of intrarenal vascular lesions is a marker of poor renal prognosis in lupus nephritis. More precisely, our data show that the somewhat poorer renal outcome observed in patients with IMCs is related to the fact that in most cases these lesions are associated with class IV lupus nephritis, and not related to the presence of the vascular lesion per se.