We studied the effect of a plain espresso coffee (171 ± 8.9 mg caffeine) which is roughly the amount in a cup of regular coffee or caffeine soda drink on fluid replacement in mildly dehydrated healthy subjects following moderate exercise, which induced dehydration to approximately 1.2% of their body weight. Subjects then rehydrated by drinking either water alone as control or caffeinated beverage plus up to 150% of the body weight they had lost. All subjects underwent both conditions. There were differences between the control and caffeine in urine specific gravity (control: 1.018 ± 0.00 vs caffeine: 1.024 ± 0.00, P = .001), urine volume (control: 200 ± 71 mL vs caffeine: 302 ± 151 mL, P = .05), and urine color (control: 2 ± 0.9 and caffeine: 4 ± 1.66, P = .00). We conclude that intake of an espresso coffee possibly impedes replacement of body fluids.