The sensitivity and positive predictive value (PPV) of fetal renal pelvic dilatation for detecting vesicoureteral reflux (VUR) can only be determined by performing voiding cystourethrography (VCUG) on all newborns in a study population. We sought to determine this for infants with a family history of reflux. From June 1989 to September 1996, 157 children (80 males) under 2 years with a family history of primary VUR in a first-degree relative had VCUG. VCUG results were compared with the fetal renal pelvic diameter on obstetric sonogram performed after 16 weeks gestation. Of the 157 infants, 33 (11 boys) had primary VUR. In those with a parent as the index case, the prevalence of reflux was 5% in boys and 35% in girls ( P=0.03). The largest renal pelvic diameter after 16 weeks was not discriminatory for VUR, but after 30 weeks gestation a 4 mm renal pelvis had a sensitivity of 33% and a PPV of 32%. The sensitivity was higher for grades 4 and 5 (75%) than for grades 1-3 reflux (17%), ( P=0.04). In conclusion, fetal renal pelvic diameter has a low sensitivity and poor predictive value for detecting VUR, but this improves a little after 30 weeks gestation.