Our aim was to correlate an immunohistochemical pattern of selected podocyte cytoskeleton-associated proteins in children diagnosed with focal segmental glomerulosclerosis (FSGS) and diffuse mesangial proliferation accompanied by glomerular immaturity (Im-DMP) with the clinical courses of both diseases. The material included 33 renal biopsies obtained from children diagnosed with DMP with or without signs of glomerular immaturity (ten and 15 participants, respectively) or FSGS (eight patients). Ezrin, podocalyxin, synaptopodin and nephrin expression was evaluated by immunohistochemical assay. A positive reaction for ezrin, podocalyxin, synaptopodin and nephrin was observed in the most superficial, continuous 'layer' of podocytes in Im-DMP patients. This distribution closely mimicked the immunohistochemical pattern observed in FSGS. The severe initial course of Im-DMP was transient. Resistance to steroids (six children) and renal insufficiency (two patients) in these subjects subsided, whilst, in the FSGS patients, the resistance to steroids recognized in all the children and the renal insufficiency diagnosed in three of them were still present. Mimicry between the immunohistochemical pattern of glomerular immaturity in DMP and focal segmental glomerulosclerosis might explain the severe initial course of this nephrotic syndrome in children. The transient clinical character of the former may also indicate that it is not a variant of FSGS.