Bone marrow changes and their relation to blood cytopenia in patients with Langerhans cell histiocytosis (LCH) have not been extensively studied to date. The aim of the present study was to characterize the bone marrow changes in LCH patients and to ascertain their relation to disease severity.

Fifty‐seven marrow samples of LCH patients were studied by conventional cytology, immunocytochemistry (ICC) and flow cytometry (FCM).

On conventional cytology there was no significant difference between LCH cases and controls with respect to cellularity, number of monocytes and progenitor cells, and presence of histiocytes and hemophagocytosis. The numbers of nucleated cells, CD34^pos cells, and CD14^pos cells on FCM did not differ, either. The CD1a staining by ICC was positive in 14/41 LCH samples, and was consistently negative in controls. FCM staining for CD1a was positive in 12/54 samples, but also in 5/35 controls. The number of the CD1a^pos cells in LCH marrows was usually very low (<10–20 cells/slide by ICC, or <0.5% of the leukocytes by FCM). The CD1a staining was more frequently positive and more pronounced in patients with severe disease.

The combination of conventional aspirate cytology with ICC (CD1a staining) appears to be the most reliable tool for bone marrow assessment in LCH. Pediatr Blood Cancer 2007;49:694–698. © 2007 Wiley‐Liss, Inc.