Neuroblastoma is the most common extracranial malignancy of childhood and advanced cases are difficult to treat. The Myc family of proteins regulates cell growth and MYCN amplified neuroblastoma is associated with a poor prognosis. Mxi1 is a member of the MAD family that inhibits N-Myc activity. Mxi0 is an alternatively spliced variant of Mxi1 with a different first exon (Exon 0) whose function has not been determined. To test the hypothesis that Mxi1 and Mxi0 differentially impact N-Myc-dependent neuroblastoma cell proliferation, we expressed Mxi1 and Mxi0 in SHEP neuroblastoma cells, and SHEP cells stably transfected to express high levels of MYCN (SHEP/MYCN). We also utilized native neuroblastoma cell lines with inducible expression of Mxi1 and Mxi0. Cell proliferation and survival were quantified using BrdU and MTT assays, respectively. Impact of Mxi1 and Mxi0 on N-Myc expression was measured by RT-PCR and immunoblot analysis. Overexpression of Mxi1 inhibits neuroblastoma cell viability. Conversely, overexpression of Mxi0 in neuroblastoma cell lines leads to increased viability, suggesting that Mxi0 is a counter-regulatory role of Mxi1. Further examination reveals that these changes are partially due to alterations in cell proliferation, with Mxi1 decreasing proliferation and Mxi0 augmenting it. Additionally, we observed that N-Myc levels decrease in response to Mxi1 expression and increase when Mxi0 is expressed. Compared with Mxi1, expression of Mxi0 enhanced chemoresistance of neuroblastoma cells to doxorubicin or etoposide. In summary, overexpression of Mxi1 in neuroblastoma cell lines leads to inhibition of N-Myc activity, while Mxi0 appears to promote it. These effects may partially be due to alterations in N-Myc expression. A better understanding of the interactions among Mxi1, Mxi0 and N-Myc and how the relative expression levels of these proteins affect neuroblastoma physiology may aid in developing more effective targeted therapies to improve outcomes in pediatric neuroblastoma patients.