Relationship among patients with LCH, ECD, and mixed histiocytosis. Principal component analysis was used to graphically describe the relationships between patients with LCH, ECD, and mixed histiocytosis. Each patient is represented by a point in a two-dimensional graph such that the distance between any two points is proportional to the strength of the association between the variables recorded in these patients (presence or absence of digestive, cutaneous, skeletal, pulmonary, CNS, pituitary, ocular, retroperitoneal, vascular, and cardiac involvements). The axes are selected as best explaining the variability of the observed data, and their scale is arbitrary. This analysis shows that LCH, ECD, and mixed histiocytosis share heterogeneous clinical patterns (because patients are widely spread within each disease) and that patients with mixed histiocytosis appear closer to patients with ECD than to patients with LCH.