In this issue of Blood, Hervier et al has identified that cooccurrence of Langerhans cell histiocytosis (LCH) and Erdheim-Chester disease (ECD) in the same patient is not a rare event. Mixed histiocytosis (MH) highlights existence of a link between distinct groups of histiocytic disorders and suggests presence of a common progenitor cell. Today, histiocytic disorders are classified into 3 groups based on clinical presentation and gross anatomic findings.