Xanthogranulomatous pyelonephritis is a rare chronic renal infection of unknown pathogenesis characterized by replacement of renal parenchyma by lipid filled macrophages frequently associated with an enlarged, non-functioning kidney and an obstructing calculus. We report a case of a 45 year old non diabetic female who presented with gradually enlarging renal mass with extensive retroperitoneal involvement and a non-functioning kidney with no evidence of obstructing stone or fat density and simulating malignancy. She was diagnosed as stage III Xanthogranulomatous pyelonephritis and managed with radical nephrectomy with favourable outcome.