Chronic dehydration stone disease. A study was made of 819 patients attending a metabolic stone clinic. A firm diagnosis was made in 708 (86%) and in 132 of these (19%) the diagnosis was thought to be chronic dehydration. The records were available for study for 87 males and 11 females in the chronic dehydration group. The mean age at presentation was 43 years. The causes of chronic dehydration were hot climate (62%), with hot occupation and low water intake almost equal in second place. In patients with a single cause of chronic dehydration, 57% also had a dietary risk factor for urolithiasis and this was most commonly high oxalate intake. Following dietary advice, the mean urinary volume increased from 1720 to 2475 ml/24 h. This was accompanied by a rise in mean urinary calcium from 6.02 to 6.96 mmol/24 h, presumably due to the calcium in the additional water drunk. Urinary oxalate did not change significantly. The mean follow-up time was 4.85 years and the stone recurrence rate was low. It was concluded that chronic dehydration is a common cause of urolithiasis; this can be treated satisfactorily by increasing water intake plus dietary advice in certain cases. 