Hirschsprung disease (HSCR) is a major cause of chronic constipation in children. HSCR can be caused by germline mutations in RET and EDNRB. Defining causality of the mutations identified is difficult and almost exclusively based on in silico predictions. Therefore, the reported frequency of pathogenic mutations might be overestimated. We combined mutation analysis with functional assays to determine the frequencies of proven pathogenic RET and EDNRB mutations in HSCR. We sequenced RET and EDNRB in 57 HSCR patients. The identified RET-coding variants were introduced into RET constructs and these were transfected into HEK293 cells to determine RET phosphorylation and activation via ERK. An exon trap experiment was performed to check a possible splice-site mutation. We identified eight rare RET-coding variants, one possible splice-site variant, but no rare EDNRB variants. Western blotting showed that three coding variants p.(Pr270Leu), p.(Ala756Val) and p.(Tyr1062Cys) resulted in lower activation of RET. Moreover, only two RET variants (p.(Ala756Val) and p.(Tyr1062Cys)) resulted in reduced ERK activation. Splice-site assays on c.1880-11A>G could not confirm its pathogenicity. Our data suggest that indeed almost half of the identified rare variants are proven pathogenic and that, hence, functional studies are essential for proper genetic counseling.