Background: Erdheim-Chester disease is a rare disease characterized by multi-organ infiltration of non-Langerhans cell histiocytes. The diagnosis can be difficult and a high index of suspicion is required to combine the constellation of multi-organ imaging features. Recent research has detected a mutated proto-oncogene BRAFV600E in over half of the cases. Case Report: We report a case of a 75-year-old woman who presented with 5 months of left knee pain. On further workup, she was found to have to multi-organ involvement for Erdheim-Chester disease. Conclusion: Erdheim-Chester disease requires a high index of suspicion along with awareness of the various clinical and radiological manifestations for accurate diagnosis.