This study (NCT01288573) investigated plerixafor's safety and efficacy in children with cancer. Stage 1 investigated the dosage, pharmacokinetics (PK), pharmacodynamics (PD), and safety of plerixafor + standard mobilization (G-CSF ± chemotherapy). The stage 2 primary endpoint was successful mobilization (doubling of peripheral blood CD34+ cell count in the 24 h prior to first apheresis) in patients treated with plerixafor + standard mobilization vs. standard mobilization alone. In stage 1, three patients per age group (2-textless6, 6-textless12, and 12-textless18 years) were treated at each dose level (160, 240, and 320 µg/kg). Based on PK and PD data, the dose proposed for stage 2 was 240 µg/kg (patients 1-textless18 years), in which 45 patients were enrolled (30 plerixafor arm, 15 standard arm). Patient demographics and characteristics were well balanced across treatment arms. More patients in the plerixafor arm (24/30, 80 met the primary endpoint of successful mobilization than in the standard arm (4/14, 28.6 p = 0.0019). Adverse events reported as related to study treatment were mild, and no new safety concerns were identified. Plerixafor + standard G-CSF ± chemotherapy mobilization was generally well tolerated and efficacious when used to mobilize CD34+ cells in pediatric cancer patients.