The objective for this study was to investigate whether MTX-induced nausea was associated with selected SNPs in candidate genes encoding MTX transporter proteins (SLCO1B1, SLCO1B3, SLC19A1, ABCB1, ABCC2), the MTHFR enzyme (MTHFR) and the 5-HT3-receptor (HTR3A, HTR3B), in children with JIA. This evidence concerns the gene MTHFR and juvenile idiopathic arthritis.