Sixty-four geriatric long-stay patients aged 65 years or older participated in the trial. All were using laxatives prior to the study. For the study laxatives the mean dose of magnesium hydroxide was 25 ml daily and for bulk-laxative 8.7 g daily. Magnesium hydroxide caused a more frequent bowel habit (13.2 vs. 10.4/4 weeks, p less than 0.001) than bulk-laxative and additional laxative bisacodyl was not needed as often as with bulk-laxative (2.3 vs. 3.3/4 weeks, p less than 0.01). Also the stool consistency was more normal during the magnesium hydroxide treatment. In two patients serum magnesium was over 1.25 mmol/l after the magnesium hydroxide treatment but there were no clinical signs of hypermagnaesemia. Our study indicated magnesium hydroxide to be more efficient than bulk-laxative in treating constipation in elderly long-stay patients.