OBJECTIVE We attempted to determine if the degree of second trimester dilatation and/or other qualitative sonographic features of the fetal urinary tract are predictive of postnatal outcome in male neonates with posterior urethral valves. METHODS We reviewed ultrasound reports and/or hard copy imaging in 17 cases of posterior urethral valves initially detected on second trimester scans (median gestation 18 weeks, range 15 to 23.5). Ultrasound appearance was categorized according to the pattern and severity of dilatation, cystic change or echogenicity of the renal cortex and presence of oligohydramnios. Outcome was reviewed at a median followup of 5.7 years (range 4.4 to 10). RESULTS Of the 17 cases there was a poor outcome in 10, including death in 4 and chronic renal failure in 6. Seven patients were alive and well with normal renal function at followup. The prognosis in cases of moderate or severe upper tract dilatation was poor. Of the 9 patients with marked prenatal hydroureteronephrosis 8 (89%) were dead or had chronic renal failure at followup. In contrast, only 2 of the 8 patients (25%) with mild upper tract dilatation or dilatation limited to the bladder had chronic renal failure at review (p = 0.05). Three cases of prenatal renal parenchymal change and 3 of subsequent oligohydramnios had a poor outcome postnatally. CONCLUSIONS The prognosis for boys with prenatally detected posterior urethral valves is closely associated with qualitative aspects of second trimester scan findings. This information may be of clinical value in the prenatal counseling of parents.