Risk factors were studied in 370 children with internal urinary system (IUS) anomalies, coming from 105,374 consecutive births of known outcome. The incidence of IUS malformations was 3.51 per 1,000 births. Diagnosis was performed prenatally in 54.4% of patients. Two hundred and fifty-two patients had isolated IUS anomalies; 118 (31.8%) of the children had at least one non-urinary malformation. Fifty-five infants (14.8%) had recognized chromosomal and non-chromosomal syndromes. The more frequent non-urinary malformations were cardiac, digestive and limb anomalies. For each case a control was studied. The following features were assessed: sex ratio, parity and previous pregnancies, parental age, residency, education, ethnic origin, length, head circumference and weight at birth, genetic and environmental factors. Odds ratio values were calculated for risk factors. Weight, length and head circumference at birth were less than in the controls and the weight of the placenta was lower. Pregnancies with IUS anomalies were more often complicated by oligo-amnios and threatened abortions. Oligo-amnios was more frequent in pregnancies in which babies had multiple malformations and recognized syndromes with IUS anomalies. One child of every three with IUS anomalies had an extra-urinary malformation, which is 12 times the incidence of such malformation in our population. There was an increase in consanguinity in the parents of our patients. The incidence of IUS anomalies in first-degree relatives was 2.9%. First-degree relatives had more non-urinary malformations than controls (7.02 vs 3.2%). Our study demonstrated the high capacity of a general ultrasound screening programme to detect fetal malformations affecting the urinary tract.