A 29-year-old Spanish man presented with chronic intestinal pseudo-obstruction, progressive external ophthalmoplegia, peripheral neuropathy, and diffuse leukoencephalopathy. This combination of clinical features is characteristic of mitochondrial neurogastrointestinal encephalomyopathy (MNGIE). Genetic analysis revealed a novel 18-base pair (bp) duplication (5044-5061 dup) in exon 8 of the thymidine phosphorylase (TP) gene. The mutation is predicted to produce a 6 amino acid insertion in the alpha-beta-domain of the protein. This 18-bp insertion in the thymidine phosphorylase gene is the first duplication mutation identified in MNGIE.