The metabolic syndrome (MetS) is a cluster of cardiovascular risk factors which has been suggested to have a possible effect on bone mass. Somewhat paradoxically, it is not clear whether this effect is protective or detrimental. Some of its components (e.g., obesity) seem to have the first type of effect and others (e.g., glucose metabolism changes) the second one. The epidemiological studies are not conclusive. Five out of six cross-sectional studies show no differences in the rate of fractures between subjects with or without MetS. In the sixth, fewer fractures were observed in patients with the syndrome. Two of three prospective studies also found fewer fractures, but the third more. Regarding the relationship of each individual component of MetS with fractures, the results - apart from obesity - are scarce or inconsistent. The relationship between MetS and bone mineral density (BMD) or bone turnover markers (BTMs) has also been addressed. Without adjusting for BMI, six out of nine studies have shown higher BMD values in MetS patients and the rest no differences. This positive effect on BMD is mainly driven by BMI, and therefore disappears after adjusting for it. The fasting plasma glucose level has been shown in general to be positively associated with BMD. Hypertension and hypertriglyceridemia showed variable results, while BTMs are decreased in MetS. Finally, there is no definite evidence about the existence of gender differences in the effect of MetS on bone. In conclusion, MetS tends to be positively associated with BMD and negatively with BTMs. No clear-cut data about fractures are available.