Between 1980 and 1986 313 patients with urinary tract infection have been investigated in a retrospective study for changes in their electrolyte and acid base metabolism. A correlation with malformations of the urinary tract has been performed. In 34% of our patients obstruction of the urinary system was demonstrated. 40% of all patients with urinary tract infection showed electrolyte disturbances in serum varying from severe to moderate. No difference in electrolyte and acid base disturbances were found between patients with and without obstructive uropathy. A remarkably diverse pattern of electrolyte changes was seen: hyponatremia singly or in various combination was observed most frequently (81%) but even hypernatremia was found. In some patients only an acid base disturbance without changes in electrolytes was seen. The pattern of hyponatremia with hyperkalemia--reported in the literature with obstructive uropathy--was observed in 6.8% of our patient population with electrolyte disturbances. This was however not restricted to patients with malformation of the urinary tract.