Erdheim-Chester disease is a rare non-Langerhans form of systemic histiocytosis of unknown origin. We describe a 45-year-old man presenting with bilateral hydronephrosis suggestive of extrinsic urinary tract obstruction. Computed tomography revealed extensive hypodense soft tissue infiltration in the retroperitoneum surrounding the kidneys. Needle biopsy of the retroperitoneal soft tissue revealed aggregates of lipid-laden histiocytes expressing CD68 but negative for CD1a and S100 protein. The diagnosis of Erdheim-Chester disease was supported by typical radionuclide bone scinitigraphic findings. Treatment with prednisolone, sirolimus, and regular ureteric stent revision was initiated to achieve adequate urinary tract drainage. To our knowledge, this is the second patient with Erdheim-Chester disease reported in Hong Kong. A high index of suspicion is required to avoid delay in the diagnosis of this rare disease.