The prognosis for infants with renal vein thrombosis has improved over the past three or four decades. At one time early thrombectomy or nephrectomy were considered necessary but more recently such procedures have been avoided and anticoagulant treatment or thrombolysis have been used. A report from the Montreal Children's Hospital (Andrew Zigman and colleagues.Journal of Pediatric Surgery2000;35:1540–2) describes 23 children diagnosed by Duplex ultrasound scan in the 1990s.  There were 12 boys and 11 girls. One child was 14 years old but the mean age of the others was 13 days and 19 were under 1 month. Only three had the classical triad of palpable abdominal mass, heavy haematuria, and thrombocytopenia. In five cases the renal vein thrombosis was bilateral and in 12 the thrombosis extended into the inferior vena cava. Five infants were considered to have had renal vein thrombosis in utero and three of these were heterozygotes for factor V Leiden. Antenatal risk factors (fetal distress, maternal diabetes, traumatic birth, antenatal steroids, pre-eclampsia, polyhydramnios, amphetamine, protein C deficiency) were identified in 15 cases and postnatal (respiratory distress, heart disease, diarrhoea, hypotension, polycythaemia, factor V Leiden) in 17.  Twelve patients were treated with heparin or enoxaparin and four of these developed long term renal impairment. All 11 not treated with heparin or enoxaparin developed long term renal impairment. Neither thrombolysis nor thrombectomy was attempted and no patient needed dialysis or transplantation. One late nephrectomy was performed for recurrent pyelonephritis.  Heparin treatment is beneficial for infants with renal vein thrombosis. Infants who suffered from renal vein thrombosis in utero should be tested for factor V Leiden.