The interrelation between transplantation antigens of the HLA system and predisposition to acute pyelonephritis (PN) were studied in 81 patients, of them 36 with acute serous PN and 45 with acute purulent PN. HLA phenotype was identified with a routine lymphocytotoxic microtest in which a home-made typing panel of HLA antiserum to antigens of A and B loci was used. 150 healthy donors were subjected to control typing. Variance in HLA frequency reliability in patients and controls was verified with chi 2 criterion for tables 2 X 2. The study demonstrated higher frequency of B8 and B14 and lower of Aw19 and B35 antigens in patients with PN versus the healthy controls. Differential analysis with regard to the clinical course of the disease revealed that predisposition to acute serous PN was associated with the occurrence of A3 and B14 antigens while A10 and B8 antigens were indicative of acute purulent PN. It has been stated that purulent PN is developing in presence of secondary deficiency of T and B immune systems. Cross-reacting antigens B14 and B8, as well as splitting antigens A10 and A25 lend more ground to varying clinical course of the disease. HLA antigen B35 is found associated with the development of acute PN resistance.