Because the finding of single heterozygous SLC22A5 mutations in a series of patients biochemically confirmed with primary carnitine deficiency suggested the presence of a second, noncoding mutation in trans rather than involvement of a different gene, we searched for mutations outside the coding region of SLC22A5. First, we Sanger sequenced SLC22A5 cDNA prepared from messenger RNA isolated from primary fibroblasts of 12 non‐related patients with biochemically confirmed primary carnitine deficiency in whom no (2 patients) or only one heterozygous mutation (10 patients) had been identified. Here, SLC22A5 is linked to systemic primary carnitine deficiency disease.