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Thrombotic microangiopathy (TMA) is a severe complication of hematopoietic stem cell transplant (HSCT). To determine incidence, risk factors and outcomes for patients who develop TMA following HSCT. All patients were prospectively screened for TMA at participating centers with daily CBC, renal panel, and blood pressure; twice weekly LDH; and weekly urine analysis, including urine protein to creatinine ratio, from the start of the preparative regimen through the first 30 days. All labs were transitioned to weekly from day +30 to +100. TMA was diagnosed if (1) pathologic evidence of TA-TMA (e.g., renal biopsy with evidence of TMA), or (2) if meeting laboratory/clinical markers diagnostic for TMA (Figure 1). Each site retrospectively reviewed the data from screened patients from their respective center, and these data were aggregated to determine outcomes. 589 consecutive patients received TMA screening at the 12 participating centers from May 1, 2016 through June 30, 2018. TMA was diagnosed in 10% (20 of 198) of autologous and 21% (85 of 391) of allogeneic HSCT recipients. Patients undergoing HSCT for an underlying immune deficiency or bone marrow failure syndrome had a significantly higher rate of TMA (p=<0.001) (Figure 2). TMA was diagnosed at a median of 32 (IQR 24-44) days post-HSCT. Patients with TMA had a higher need for intensive care admission (46% vs. 17%, p=0.0001); increased incidence of respiratory failure (19% vs. 7%, p=0.0001); and increased incidence of bloodstream infections (44% vs. 22%, p=0.0001) in the first 100 days versus patients not diagnosed with TMA. Patients with TMA averaged significantly more days inpatient during the first 100 days than those without TMA (65 +/-27 vs. 41 +/-22 days; p=0.001). Further, patients with TMA averaged significantly more days in the ICU during the first 100 days than those without TMA (10 +/-20 vs. 3 +/-11 days; p=0.001). Six-month non-relapse mortality was significantly higher in the TMA group (19/105,18% vs. 19/484,4%; p=0.0001). Finally, six-month overall survival was significantly decreased in patients with TMA (84/105, 80% vs. 453/484,94%; p=0.0001) (Figure 3). In patients diagnosed with TA-TMA, 49 (47%) were treated with eculizumab and 3 (3%) received plasmapheresis. In this multi-center cohort we demonstrate a high incidence of TMA after pediatric HSCT through prospective screening. Patients with TA-TMA have higher morbidity and mortality when compared to patients without TA-TMA. [ABSTRACT FROM AUTHOR]