To test the hypothesis that increased fluid intake increases the urine free cortisol, we prescribed 5 liters of fluid intake per day or a normal fluid intake according to a randomized cross-over design in six normal volunteers. Each period lasted 5 days, with a 2-day washout period of normal fluid intake between the two periods. Urine free cortisol, 17-hydroxycorticosteroids, and creatinine were measured daily during each study period, and the average value over each 5-day period was calculated for each subject. High fluid intake caused a significant increase in the mean urine free cortisol [126 +/- 33 (SD) vs. 77 +/- 18 micrograms/day, P < 0.005]. The frequency of urine free cortisol results that exceeded the upper normal limit of 95 micrograms/day was also much higher during high fluid intake (23/30 vs. 6/30, P < 0.005). By contrast, urine 17-hydroxycorticosteroids (high fluid vs. normal fluid: 5.3 +/- 1.5 vs. 5.0 +/- 1.7 mg/day, respectively, P = not significant) and urine creatinine (1.51 +/- 0.48 vs. 1.45 +/- 0.37 g/day, P = not significant) did not differ between the two study periods. We conclude that high fluid intake (5 liters/day) increases free cortisol excretion without an increase in urine 17-hydroxycorticosteroids. Thus, mild to moderate increases in urine cortisol excretion may not indicate hypercortisolism in individuals who have a high fluid intake and urine volume.