This Phase I study investigated the recommended Phase II dose (RP2D) of inotuzumab ozogamicin (InO), a CD22-directed antibody-drug conjugate, in pediatric patients with multiple-relapsed/refractory (R/R) CD22-positive acute lymphoblastic leukemia (ALL). Patients (≥1-<18 years) received three InO doses (Days 1, 8, 15) per course. Dose-escalation was based on dose-limiting toxicities (DLT) during Course 1 . Dose level 1 (DL1)=1.4 mg/m2 (0.6-0.4-0.4 mg/m2); DL2=1.8 mg/m2 (0.8-0.5-0.5 mg/m2). Secondary endpoints included safety, anti-leukemic activity, and pharmacokinetics. Twenty-five patients (23 evaluable for DLT) were enrolled. In Course 1, first cohort, 1/6 (DL1) and 2/5 (DL2) patients experienced DLTs; subsequent review considered DL2 DLTs to be non-dose-limiting. Dose was de-escalated to DL1 while awaiting protocol amendment to re-evaluate DL2 in a second cohort, where 0/6 (DL1) and 1/6 (DL2) patients had a DLT. Twenty-three patients experienced Grade 3-4 adverse events; hepatic sinusoidal obstruction syndrome was reported in two patients after subsequent chemotherapy. Overall response rate after Course 1 was 80% [95% CI: 59-93%] (20/25 patients; DL1=75% [43-95%], DL2=85% [55-98%]); 84% [60-97%] of responders obtained minimal residual disease-negative CR; 12-month overall survival was 40% [95% CI: 25-66%]. Nine patients received hematopoietic stem cell transplant or chimeric-antigen receptor T-cells after InO. InO median maximum concentrations were comparable to simulated adult concentrations. InO was well tolerated, demonstrating anti-leukemic activity in heavily pre-treated children with CD22-positive R/R ALL. RP2D was established as 1.8 mg/m2/course, as in adults.