A 27-year-old man consulted with clinical and radiological features of chronic erosive oligoarthritis of large joints (hips and knee), associated with diffuse lymph-node enlargement and diabetes insipidus. Lymph-node biopsy provided the diagnosis of systemic Langerhans’ cell histiocytosis, for which synovial involvement remains a diagnostic challenge. Infectious diseases search and immunological tests were all negative. Skeleton radiographs, hip and cerebral magnetic resonance imaging showed, respectively, erosive arthritis of the hips and stigmates of pituitary-stalk involvement. Hip-synovium biopsy exhibited the main histological features of Erdheim–Chester disease, a non-Langerhans’ cell histiocytosis. An extensive literature review found that Langerhans’ cell histiocytosis and non-Langerhans’ cell histiocytosis (mainly Erdheim–Chester disease) coexistence is rare and synovial involvements in them even more so, these latter presenting mainly as large joint monoarthritis. The absence of typical clinical and radiographic signs of Erdheim–Chester disease led to consideration of the rheumatologic diagnosis of unclassified non-Langerhans’ cell histiocytosis (or Erdheim–Chester disease-type) oligoarthritis, associated with multiorgan Langerhans’ cell histiocytosis. The differential diagnosis of large joint erosive arthritis should then include both entities, particularly when multiorgan manifestations are present. Non-Langerhans’ cell histiocytosis synovial involvements responded poorly to vinblastine and corticosteroids, while Langerhans’ cell histiocytosis involvements responded completely but transiently. Both entities regressed under cladribine, with only mild relapses of the non-Langerhans’ cell histiocytosis involvements.