The characteristics of chronic pyelonephritis are studied in 37 patients out of a total of 53 patients with proved renal polycystosis. A group of 71 patients with chronic pyelonephritis selected at random are used as a control group. The frequency of chronic pyelonephritis among the patients with renal polycystosis is 69.8%. The difference between the mean age of the patients with renal polycystosis and chronic pyelonephritis and the patients with renal polycystosis without chronic pyelonephritis is 8.6 years. A significant difference is established between these two groups of patients concerning the frequency of symptomatic hypertension--89.2% for the patients with renal polycystosis and chronic pyelonephritis and 45% for the patients with uncomplicated renal polycystosis. A similar difference is established also for the renal failure--respectively 64.9% and 37.5%. The frequency of hypertension and chronic renal failure is lower in the control group of patients. 59% of the patients with renal polycystosis and chronic pyelonephritis have significant bacteriuria, E. coli and Proteus being the most frequently isolated bacteria but Pseudomonas shows the highest drug resistance. The isolated bacteria are most sensitive to nitroxoline and aminoglycoside antibiotics.