We performed this cross-sectional case control study to investigate the association between low serum magnesium levels and cognitive impairment in hypertensive hospitalized patients. The study was carried out in general medical care units at 81 hospitals participating in the Gruppo Italiano di Farmacovigilanza nell'Anziano (GIFA) study throughout Italy, and a total of 1058 patients with diagnoses of hypertension at the discharge were studied. The occurrence of cognitive impairment at discharge was the primary outcome of our study. Cognitive impairment was ascertained using the Hodkinson Abbreviated Mental Test (HAMT). Sociodemographic variables, body mass index, laboratory parameters, comorbidity, use of antihypertensive drugs and number of drugs were considered as potential confounders. Twenty-nine percent of the selected hypertensive patients were classified as having cognitive impairment. In univariate analysis, older age, female sex, and low educational level showed a significant trend for association to cognitive impairment. Moreover the proportion of subjects with cognitive impairment decreased with increasing alcohol consumption, and the prevalence of ex smokers and smokers was significantly lower in patients with cognitive impairment. The lower tertiles for serum albumin and creatinine clearance were more frequent among patients with cognitive impairment, and the lower tertile for serum magnesium levels was significantly more frequent in these patients. Number of drugs was slightly lower in cognitively impaired patients, while number of diagnoses and length of hospital stay were higher in these subjects. In the multivariate logistic regression analysis cognitive impairment decreased with increasing education level (highest education: OR 0.11; 95 per cent CI 0.05-0.25). The lower tertile for serum albumin (< 3.5 g/dl) was significantly associated to cognitive impairment (OR 2.14; 95 per cent CI 1.31-3.49), as well as the lower tertiles for serum magnesium (0.74-0.86 mmol/L: OR 1.54; 95 per cent CI 1.06-2.22; < 0.74 mmol/L: OR 1.75; 95 per cent CI 1.13-2.72]. Our results demonstrate the existence of a significant association between magnesium imbalance and cognitive impairment. These data suggest that the assessment of magnesium status may be of some relevance in hypertensive subjects with cognitive disorders.