The combination of 2-chlorodeoxyadenosine (2-CDA) and cytosine arabinoside (Ara-C) has been shown to be effective in children with refractory Langerhans cell histiocytosis (LCH). We have treated 5 patients with recurrent LCH with 2-CDA/Ara-C chemotherapy and closely followed immune and hematopoietic function. These patients display a decline in the absolute CD4, CD8, and natural killer cell number, decrease in the CD4/CD8 ratio. Septic events, including pneumocystis infection were present after most of the treatment courses (15/21). These data suggest that 2-CDA /Ara-C, should be considered in resistant and relapsed pediatric patients with LCH with high-risk multiorgan involvement. Consequent profound prolonged combined immune deficiency and myelosupression should be anticipated.