The microbiome and immune system of the digestive tract are highly important in both health and disease. Exclusive enteral nutrition (EEN) is a common anti-inflammatory treatment in children with Crohn's disease in the European countries, and the mechanism is most likely linked to changes in the intestinal microbiome. In the present study, EEN was given in two treatment periods several months apart to a patient with very severe, disabling juvenile idiopathic arthritis (JIA), with a remarkable clinical response as the result. The aim of the present study was to study how the EEN treatment influenced the microbiome and metabolome of this patient. Fecal samples from before, during, and between treatments with EEN were studied. The microbiome was analyzed by sequencing of 16S rRNA amplicons using Illumina MiSeq, and the metabolome was analyzed using nuclear magnetic resonance. The microbiome changed markedly from treatment with EEN, with a strong reduction of the Bacteroidetes phylum. Metabolic profiles showed clear differences before, during, and between treatment with EEN, where butyrate, propionate, and acetate followed a cyclic pattern with the lowest levels at the end of each treatment period. This patient with JIA showed remarkable clinical improvement after EEN treatment, and we found corresponding changes in both the fecal microbiome and the metabolome. Further studies are needed to explore the pathophysiological role of the intestinal canal in children with JIA.