BACKGROUND Membranous nephropathy (MN) is the most common cause of nephropathy in adults. The diagnosis is based on characteristic light microscopic, electron microscope and immunofluorescence (IF) findings. In early MN, the light microscopic findings may be difficult to differentiate from minimal chain disease. In the absence of fresh frozen tissue for IF, immunohistochemistry with C4d aids in the diagnosis. METHODS A total 48 cases of MN diagnosed on renal biopsy were analyzed. The formalin fixed paraffin embedded tissues were stained with routine hematoxylin and eosin stains along with periodic acid-Schiff and silver methenamine stains to highlight the basement membrane. Fresh frozen tissues were available for IF in 40 cases. Immunostaining with C4d was done on paraffin-embedded sections by polymer-Horse Radish Peroxidase (HRP) technique using polyclonal antiserum to C4d (Biogenex, India). RESULTS There were 25 cases of idiopathic MN, 17 cases of Class V lupus nephritis and 2 cases were secondary to hepatitis C infection with cirrhosis. The glomerular basement membrane (GBM) was diffusely thickened with formation of spikes in 28 cases. In 11 cases the capillary loops were rigid but spikes were not seen and in 9 cases there was no apparent thickening of the basement membrane. All the cases showed diffuse positivity for C4d along the GBM. CONCLUSIONS C4d is a reliable method to establish the diagnosis of MN and also a sensitive marker of complement activation reflecting the pathogenesis of MN.