The study investigated the serum levels of magnesium and red blood cell distribution width (RDW) values and their relationship with polysomnographic parameters in patients with Obstructive sleep apnoea syndrome (OSAS). Serum RDW values were significantly elevated and magnesium levels were significantly decreased in OSAS. Also, RDW was positively and magnesium was negatively correlated with polysomnographic parameters. To our knowledge, this was the first study investigating the association between RDW and Mg levels in the same patient population.