The article presents the data on the frequency of phenotypic and clinical signs of undifferentiated connective tissue dysplasia (UCTD) in infants with congenital malformations of the urinary system (CMUS) complicated by pyelonephritis. The considerable incidence of connective tissue dysfunction among the examined children has been shown, especially among the children with anomalies of the renal tissue formation and differentiation. The authors draw the conclusion about the necessity of early verification of UCTD among young children with pyelonephritis against the CMUS. This will allow ground and perform metabolic correction of connective tissue dysfunction, which should raise the treatment efficiency in such children