A prospective study was undertaken to define the clinical features, natural history and etiology of post-obstructive diuresis. Studied in detail were 8 patients with a massive diuresis after relief of urinary tract obstruction. We found that urea mediated osmotic diuresis, natriuresis owing to elimination of retained sodium from the obstructed phase, tubular defects in sodium reabsorption, renal unresponsiveness to antidiuretic hormone and iatrogenic factors may play varying roles in each individual diuresis. Overzealous fluid replacement should be avoided.