Assessment of dry weight in pediatric hemodialysis (HD) patients is difficult, since small fluid shifts may result in dialysis-associated morbidity (DAM) and children may not verbalize complaints. Achieving dry weight is critical since chronic fluid overload can result in hypertension and left ventricular hypertrophy. To determine if non-invasive monitoring of hematocrit (NIVM) is useful in preventing DAM in pediatric HD patients, we reviewed 200 HD treatments performed with or without NIVM (no NIVM). DAM was defined as an "event" (e.g., hypotension, headache, cramping) that required nursing intervention. Patient age, weight, and gender were similar in both groups. Desired ultrafiltration was obtained in both groups. The event rate was lower in NIVM than no NIVM for all treatments (0.22 vs. 0.3, P = 0.07) and significantly lower in patients < 35 kg (0.25 vs. 0.47, P = 0.01). The second event rate (fraction of treatments with one event that had a subsequent event occurring at least 15 min later) was lower with NIVM (P < 0.01). For the NIVM group, events in the first 90 min occurred when blood volume changed > 8% per hour; 71% of events (43/60) at 90-240 min occurred when blood volume changed > 4% per hour. NIVM decreases DAM in pediatric HD patients, especially those < 35 kg. Ultrafiltration with blood volume change < 8% per hour is safe in the 1st h and < 4% after 1 h reduces DAM in children.