Neonatal renal vein thrombosis (RVT) is a well-recognized clinical entity which is associated with serious morbidity. However, current information regarding RVT has been restricted to case reports and small case series. In this study, it was our objective to describe patient demographics, clinical presentation, location and risk factors of RVT. For our study design, we looked at a case series of 72 neonates with RVT referred to the 1-800-NO-CLOTS consultation service between 9/1996 and 8/2001. Data on age, gender, associated conditions, prothrombotic disorders, family history, location of the thrombosis, diagnostic techniques, and treatment were prospectively recorded using a standardized form. Our results show that RVT affected males (65%, CI 52-76%) significantly more often than females (35%, CI 24-48%). Median age at presentation was 2 days (0-21 days). RVT was unilateral in 72% (left side: 67%, CI 49-81%; right side: 33%, CI 19-51%), and bilateral in 28%. The majority (83%) had at least one associated condition: Prematurity (54%), central venous lines (17%), a diabetic mother (13%), asphyxia (6%), infections (6%). Prothrombotic testing was performed in 21 neonates. Activated protein C resistance was found in 8 children (38%), other defects in three. This is the largest case series of neonatal RVT to date. Data from the study show that i) male infants are affected twice as often as females and ii) there appears to be a left-sided predominance of neonatal RVT. Neonatal RVT is only infrequently associated with the presence of a catheter as compared to thrombosis at other sites. The majority of infants have associated conditions with prematurity being most frequent. A small subset of neonates were screened for prothrombotic abnormalities and 50% of the children screened were positive.