Investigations were carried in 70 patients with ulceration of lower extremities during chronic aortal obstruction. The patients were subjected to angiography, ultrasonographic determinations of pressure and quantitative and qualitative bacteriological studies. Quantitative studies were carried on by taking a smear from one square centimeter of ulceration surface. The isolated microorganisms were cultured on standard media for bacteria growing in aerobic conditions. Cultures for detection of anaerobic bacteria were performed on thioglycolate media (qualitative study). It was found that clinical symptoms of infection occur when for 1 cm2 of ulceration surface more than 5 x 10(4) of bacterial cell are present. In all patients reconstructive procedures improving blood flow were performed. Comparison of consecutive bacteriological studies performed before and after surgery permitted for evaluation of dynamics of infections and enabled choice of proper treatment and evaluation of its effectiveness.