Pleiotrophin (PTN) and midkine (MK) are members of a new family of neurotrophic factors whose expression is developmentally regulated. PTN also transforms NIH 3T3 cells, and MK is mitogenic to certain cell lines. Neuroblastomas are tumors derived from neural crest cells, and recent studies have revealed that the biology of these tumors is at least partly regulated by neurotrophic factors and their receptors. To examine the expression of PTN and MK in neuroblastoma, we analyzed their mRNA expression in 72 primary neuroblastomas and 11 neuroblastoma cell lines as well as other tissues and cell lines. PTN is highly expressed in favorable neuroblastomas (stages I, II, and IV-S, n = 44), whereas it is expressed at a significantly lower level in advanced tumors (stages III and IV, n = 28, P = 0.003). PTN is not expressed in either aggressive neuroblastomas with N-myc amplification or in neuroblastoma cell lines. Moreover, the expression pattern of PTN was similar to that of TRK-A, the high affinity receptor for nerve growth factor, in that it is correlated with a favorable prognosis (P < 0.004). In contrast, MK is highly expressed in almost all primary neuroblastomas and cell lines and showed no correlation with disease stage or N-myc amplification. These results suggest that differential expression of PTN and MK may have an important role in regulating growth and differentiation of neuroblastomas.