The enteric nervous system is derived from neural crest cells that migrate from the caudal hindbrain and colonise the gut. Failure of neural crest cells to fully colonise the gut results in an "aganglionic zone" that lacks a functional enteric nervous system over a variable length of the distal bowel, a condition in human infants known as Hirschsprung's disease. The variability observed in the penetrance and severity of Hirschsprung's disease suggests a role for modifier genes. Clinical studies have identified a population of Hirschsprung's patients with mutations in L1CAM that also have a common polymorphism in RET, suggesting a possible interaction between L1CAM and RET. Therefore, we examined whether L1cam could interact with Ret, its ligand Gdnf, and a known transcriptional activator of Ret, Sox10. Using a two-locus complementation approach, we show that loss of L1cam in conjunction with a heterozygous loss of Ret or Gdnf did not result in aganglionosis. However, L1cam did interact with Sox10 to significantly increase the incidence of aganglionosis. We show that an interaction between L1cam and Sox10 significantly perturbs neural crest migration within the developing gut, and that neural crest cells undergo excessive cell death prior to gut entry. Finally, we show that Sox10 can regulate the expression of L1cam. Thus, L1cam can act as a modifier gene for the HSCR associated gene, Sox10, and is likely to play a role in the etiology of Hirschsprung's disease.