Evidence suggests a role of Mg and the ratio of Ca:Mg intakes in the prevention of colonic carcinogenesis. The association between these nutrients and oesophageal adenocarcinoma – a tumour with increasing incidence in developed countries and poor survival rates – has yet to be explored. The aim of this investigation was to explore the association between Mg intake and related nutrients and risk of oesophageal adenocarcinoma and its precursor conditions, Barrett’s oesophagus and reflux oesophagitis. This analysis included cases of oesophageal adenocarcinoma (n 218), Barrett’s oesophagus (n 212), reflux oesophagitis (n 208) and population-based controls (n 252) recruited between 2002 and 2005 throughout the island of Ireland. All the subjects completed a 101-item FFQ. Unconditional logistic regression analysis was applied to determine odds of disease according to dietary intakes of Mg, Ca and Ca:Mg ratio. After adjustment for potential confounders, individuals consuming the highest amounts of Mg from foods had significant reductions in the odds of reflux oesophagitis (OR 0·31; 95 % CI 0·11, 0·87) and Barrett’s oesophagus (OR 0·29; 95 % CI 0·12, 0·71) compared with individuals consuming the lowest amounts of Mg. The protective effect of Mg was more apparent in the context of a low Ca:Mg intake ratio. No significant associations were observed for Mg intake and oesophageal adenocarcinoma risk (OR 0·77; 95 % CI 0·30, 1·99 comparing the highest and the lowest tertiles of consumption). In conclusion, dietary Mg intakes were inversely associated with reflux oesophagitis and Barrett’s oesophagus risk in this Irish population.