We retrospectively evaluated reticulin fiber density (RFD) in 166 diagnostic bone marrow (BM) biopsies and 62 biopsies obtained at treatment day 29 from children with acute lymphoblastic leukemia (ALL). Patients with B-cell precursor (BCP)-ALL showed higher RFD as compared to patients with T-cell ALL (P<0.001). RFD correlated negatively with white blood cell count (P=0.008) in BCP-ALL patients. Patients with high-hyperdiploid ALL (51-61 chromosomes), no high-risk criteria and low RFD showed a favorable outcome when compared to similar patients with high RFD (P=0.002). In BCP-ALL patients, RFD at diagnosis correlated to the levels of minimal residual disease (MRD) analyzed by flow cytometry on treatment day 29 (P=0.001). Accordingly, patients with MRD > or = 10(-4) presented higher RFD at diagnosis compared to patients with MRD < 10(-4) (P=0.003). BCP-ALL patients with low RFD at diagnosis and a rapid reduction of RFD on day 29 had a favorable outcome compared to patients with the same baseline RFD level at diagnosis but a slow RFD reduction (P=0.041). To our knowledge, these findings are novel and may indicate BM fibrosis as a new valuable prognostic marker in childhood ALL. Expanded use of BM biopsy both at diagnosis and during follow-up is suggested.