Background <p>Mixed histiocytosis (MH), the coexistence of two or more histiocytic disorders in the same patient, is rare and poorly understood. Langerhans cell histiocytosis (LCH) is a clonal myeloid disorder characterized by infiltration of pathological antigen-presenting cells with morphological and phenotypic resemblance to Langerhans cells. Activating mutations of the MAPK/ERK signaling cascade represent the principal driver of LCH pathogenesis and are also central to Erdheim-Chester disease (ECD).</p> Methods <p>We present a 34-year-old patient diagnosed with a mixed form of LCH and ECD involving the kidneys, skin, bones, ENT organs, eyes, and central nervous system. The clinical phenotype included diabetes insipidus, chronic otitis externa, and suspected hereditary anemia and hemochromatosis. Tissue samples were examined using histochemical, immunohistochemical, and immunofluorescence methods, along with whole-exome sequencing (WES) of blood and skin. The patient demonstrated a partial clinical and laboratory response to targeted therapy with trametinib and desmopressin.</p> Results <p>Comparison of WES data from blood and skin revealed 71,953 shared variants consistent with likely germline origin, 3,081 somatic variants in skin, and 2,633 in blood. A low-level mosaic <i>BRAF</i> V600E mutation (variant allele frequency 1%) was identified in skin, in addition to pathogenic germline variants in <i>STEAP3</i> (c.523–2&#xa0;A &gt; T, NM_182915.3) and <i>SLC40A1</i> (c.332T &gt; A, p.Met111Lys, NM_014585.6). Immunofluorescence of bone marrow trephine biopsy demonstrated heterogeneous cellular populations: sparse tryptase-positive mast cells with minimal intercellular contacts; Vimentin⁺CD11b^–CD34⁺ (0.68%) and Vimentin⁺CD11b⁺CD34^– (7.78%) stromal subsets; and a predominance of CD68⁺ cells (55.61%) over S100⁺ cells (43.02%). Double-positive CD68⁺S100⁺ cells were infrequent (2.174%), consistent with transitional stages of Langerhans cells toward LCH cells (up to 4.81%). This hybrid microenvironment paralleled the patient’s systemic inflammatory activity (elevated acute-phase reactants), multi-organ involvement, and renal interstitial fibrosis/tubular atrophy with impaired kidney function, without serological or histological evidence of vasculitis.</p> Conclusion <p>This case illustrates the histological heterogeneity and hybrid nature of MH, where canonical dendritic LCH cells coexist with macrophage-rich ECD-like infiltrate. An integrated approach involving a multidisciplinary clinical team, thorough molecular genetic testing, and the application of targeted therapy is essential for improving prognosis and quality of life in young patients.</p>