The Histiocyte Society divides the histiocytic disorders into three classes: Langerhans-cell histiocytosis (LCH) (class I), non-Langerhans-cell histiocytosis (class II), and malignant histiocytic disorders (class III). Based on recent minor revisions of the classification, the three major groups are now designated as (i) dendritic cell-related disorders (of which LCH is by far the most common), (ii) macrophage-related disorders, and (iii) malignant disorders.

Pulmonary Langerhans-cell histiocytosis (PLCH), previously termed histiocytosis X or pulmonary eosinophilic granuloma, is characterized by infiltration in lung parenchyma of CD1a-positive histiocytes—Langerhans cells. Approximately 15% of patients with PLCH have LCH with multiorgan involvement. The histopathologic findings in both cases are the same. Almost all patients with PLCH are adults, and more than 90% have a history of smoking. Cigarette smoking is thought to play an important causal role in PLCH. Many patients have complete resolution of disease after smoking cessation. A small proportion of patients have progressive disease with widespread honeycombing and, ultimately, death.