Renal biopsy specimens from five patients with Takatsuki's syndrome were studied. Proteinuria and hematuria were modestly positive in four cases and three cases, respectively. One case demonstrated early azotemia, while four cases revealed normal serum creatinine levels. All cases displayed glomerular enlargement and lobulation, mesangial cell proliferation, and double contour structure, simulating membranoproliferative glomerulonephritis. It is noteworthy that mesangial loosening was also detected in the renal biopsy specimens of this study. This change was present diffuse-globally in the well-developed cases and focal-globally or segmentally in the developing cases. In addition, various morphologic expressions were observed, ie, microaneurysms congested with RBCs, tentatively termed mesangial hyaline thrombi, nodule-like lesions, and tiny to small-sized mesangiolytic lesions. Since all these features could be explained by low-grade mesangiolysis, the renal alteration associated with this syndrome was termed mesangiolytic glomerulonephritis. Humoral factor(s) also toxic to the mesangium were implicated.