Provision of kidney replacement therapy (KRT) to manage kidney injury and volume overload in critically ill neonates and small children is technically challenging. The use of machines designed for adult-sized patients, necessitates large catheters, a high extracorporeal volume relative to patient size, and need for blood priming. The Aquadex FlexFlow System (CHF Solutions Inc., Eden Prairie, MN) is an ultrafiltration device designed for fluid removal in adults with diuretic resistant heart failure. It has an extracorporeal volume of 33 ml, which can potentially mitigate some complications seen at onset of KRT in smaller infants. In this multicenter, retrospective case series of children who received KRT with an ultrafiltration device (n=119 admissions, 884 circuits), we report demographics, circuit characteristics, complications, and short- and long-term outcomes. Patients were grouped according to weight (<10, 10-20, and >20 kg), and received one of three modalities: slow continuous ultrafiltration, continuous venovenous hemofiltration (CVVH), or prolonged intermittent KRT. Our primary outcome was survival to end of KRT. Treatment patterns and outcomes varied between the groups. In patients who weighed <10 kg, the primary indication was AKI in 40%, volume overload in 46%, and ESKD in 14%. These patients primarily received CVVH (66%, n=48) and prolonged intermittent KRT (21%, n=15). In the group weighing >20 kg, volume overload was the primary indication in 91% and slow continuous ultrafiltration was the most common modality. Patients <10 kg had lower KRT survival than those >20 kg (60% versus 97%), more volume overload at onset, and received KRT for a longer duration. Cardiovascular complications at initiation were seen in 3% of treatments and none were severe. Complications during therapy were seen in 15% treatments and most were vascular access-related. We report the first pediatric experience using an ultrafiltration device to provide a range of therapies, including CVVH, prolonged intermittent KRT, and slow continuous ultrafiltration. We were able to initiate KRT with minimal complications, particularly in critically ill neonates. There is an unmet need for devices specifically designed for younger patients. Having size-appropriate machines will improve the care of smaller children who require kidney support. Copyright © 2019 by the American Society of Nephrology.