Dehydration is commonly believed to result in headache, but the effectiveness of increasing the water intake in patients who frequently suffer from headaches has not been studied thus far. In a pilot study, we examined the possible effects and feasibility of increased water intake in headache patients. Eighteen headache patients (all had migraine, two also had tension-type headache) were randomly allocated to placebo medication, or the advice to additionally drink 1.5 l of water per day, for a period of 12 weeks. Effect measurements consisted of a 2 weeks headache diary and the Migraine Specific Quality of Life (MSQOL) questionnaire. The advice to increase the daily fluid intake by 1.5 l increased the fluid intake in the intervention group by approximately 1 l. This reduced the total hours of headache in 2 weeks by 21 h (95% CI: −48 to 5). Mean headache intensity decreased by 13 mm (95% CI: −32 to 5) on a visual analogue scale (VAS). The effects on MSQOL, number of headache episodes, and medication seemed to be small. The data of the present study suggest a reduction in the total number of hours and intensity of headache episodes after increased water intake. Our results seem to justify larger scaled research on the effectiveness of increased water intake in headache patients.