Cross-sectional studies have demonstrated that obstructive sleep apnoea (OSA) and metabolic syndrome (MetS) are often associated, but whether a temporal relationship exists is unknown. We aimed to investigate the effect of OSA on the risk of developing MetS in the general population.A prospective study was conducted combining two population-based samples: Episono (Brazil) and HypnoLaus (Switzerland). MetS was assessed according to unified criteria. Polysomnography (PSG) was performed at baseline and follow-up in Episono, and at baseline in HypnoLaus. OSA was defined according to the apnoea-hypopnoea index as mild (≥5- <15 events h ^-1 ) and moderate-to-severe (≥15 events·h ^-1 ). We included 1853 participants (mean±sd age 52±13 years, 56% female) without MetS at baseline.After mean±sd 6±1 years, 318 (17.2%) participants developed MetS. Moderate-to-severe OSA was independently associated with incident MetS (OR 2.58, 95% CI 1.61-4.11) and increased the number of MetS components from baseline to follow-up through mediation of the percentage of time with arterial oxygen saturation <90%. Subset analysis in Episono confirmed that the increase in this parameter between baseline and follow-up PSGs represented a risk factor for incident MetS (OR 1.42, 95% CI 1.04-1.95, for each 10% increase).OSA is independently associated with an increased risk of developing MetS through mediation of nocturnal hypoxaemia in the general population.