OBJECTIVES: To study whether there are any negative or positive effects of 6 months of increased fluid intake in reasonably healthy elderly men. DESIGN: Randomized trial. SETTING: Community-based. PARTICIPANTS: One hundred forty-one healthy participants aged 55 to 75. INTERVENTION: One group was given the advice to increase their daily fluid intake by 1.5 L of water; the other group was given placebo medication (8 mL inactive syrup per day). MEASUREMENTS: At 6 months blood sodium, glomerular filtration rate (GFR), blood pressure, and quality of life (QOL) were measured. The changes in water turnover were measured using deuterium. RESULTS: Most subjects did not manage to increase their fluid intake by 1.5 L. The average increase in the intervention group was approximately 1 L. Twenty-four-hour water turnover in the water group was 359 mL (95% confidence interval=171-548) higher than that of the control group at 6-month follow-up. Blood pressure, sodium level, GFR, and QOL did not change significantly in either group during the intervention period. In addition, the cases reporting a worsening on the effect measures were equally distributed over the two study groups. CONCLUSION: The advice to increase fluid intake by 1.5 L had no negative effects in reasonably healthy men aged 55 to 75.