We report the clinical history of an adult patient that debuted during adolescence with sharp episodes of arthritis several joints, with affectation of big articulations of superior and inferior members, in absence of renal symptoms. An X-ray showed calcification of the menisci in both knees, the symphysis of the pubis and of other joints. When the presence of hypomagnesaemia was demonstrated, we observed that he also had hypokalemia, hypocalciuria, increment of the urinary elimination of magnesium, slightly high levels of renin, dilution defect and a moderate reduction of the NaCl tubular distal reabsorption, all compatible with the diagnosis of Gitelman syndrome. We sequenced the exons and intron flanking regions of the SLC12A3 gene, encoding the thiazide-sensitive Na-Cl cotransporter, and showed that the patient is homozygous for a new mutation in intron 7. This mutation consisted of a G to A transition at position +1 of the donor splice site of intron 7.