Expression of the c-kit proto-oncogene product in neuroblastomas has been reported, but its clinical relevance is unclear. We determined the expression of c-kit by immunohistochemistry in a series of 155 neuroblastomas with long-term follow-up. The specificity of the reaction was verified by Western blot analysis and quantitative RT-PCR, and exon 11 of the kit gene was screened for mutations by PCR and capillary electrophoresis. No mutations were detected, and transcription of the kit gene correlated with protein expression. c-kit expression was associated with lower tumor stages and a low rate of MYCN amplification. More importantly, it coincided with tumor differentiation (P<0.0001), and portended a favorable outcome with a relative risk of 0.18 (P<0.0001). In a multivariate analysis of event-free survival, loss of c-kit (relative risk 4.25, P<0.0001) was an independent prognostic factor next to INSS stage 4 and before MYCN amplification. It is concluded that c-kit is transcriptionally regulated in neuroblastomas. Its expression likely identifies a subset of neuroblastomas with conserved capacity for differentiation, which may represent the embryonal variety of the disease. Assessment of c-kit may improve prognostic models for neuroblastoma and provide a basis for new therapy concepts.