A 61-year-old woman was found to have a right flank mass. Radiologically, the mass was thought to be of renal origin. Surgically, however, the tumorous lesion adhered tightly not only to the cortical surface but extended to the retroperitoneum and psoas muscle. Pathological examination confirmed the lesion to be malacoplakia which was characterized by the presence of von Hansemann cells and Michaelis-Gutmann bodies. Despite the effort made to focus on the initiating site of the tumor, it was considerably difficult to decide whether it originated from either an extremely superficial part of the renal cortex or renal capsule or retroperitoneum itself. Histochemical and electron microscopical investigations on the von Hansemann cells and Michaelis-Gutmann bodies were made. The pathogenesis of malacoplakia was briefly discussed with particular emphasis on its possible occurrence in the patients under various therapeutic regimes.