Inclusion Criteria:

• Before any study-specific procedure, the appropriate written informed consent must be obtained. In addition to the written informed consent, the assent of the child from those subjects capable of providing assent must also be obtained if requested by the IRB/IEC.
• Diagnosis of ITP according to The American Society of Hematology (ASH) Guidelines at least six months prior to screening
• Age ≥ 12 months and < 18 years at enrollment
• The mean of two platelet counts taken during the screening period must be ≤ 30 x 10^9/L with no single count >35 x 10^9/L
• A serum creatinine concentration ≤ 1.5 times the laboratory normal range (for each age category)
• Adequate liver function; serum bilirubin ≤ 1.5 times the laboratory normal range
• Hemoglobin >10.0 g/dL

Exclusion Criteria:

• Known history of a bone marrow stem cell disorder (any abnormal bone marrow findings other than those typical of ITP must be approved by Amgen before a subject may be enrolled in the study)
• Known history of venous or arterial thrombotic or thromboembolic event
• Known history of congenital thrombocytopenia
• Known history of malignancy except basal cell carcinoma
• Known history of hepatitis B, hepatitis C, or HIV
• Known history of systemic lupus erythematosus, Evans Syndrome, or autoimmune neutropenia
• Known positive lupus anticoagulant or history of antiphospholipid antibody syndrome
• Known history of Disseminated Intravascular Coagulation, Hemolytic Uremic Syndrome, or Thrombotic Thrombocytopenic Purpura
• Currently receiving any treatment for ITP except for corticosteroids
• IV Ig or anti-D Ig within two weeks prior to the screening visit
• Rituximab (for any indication) within 14 weeks before the screening visit or anticipated use during the time of the proposed study
• Splenectomy within eight weeks of the screening visit
• Received hematopoietic growth factors including IL-11 (oprelvekin) within four weeks before the screening visit
• Received any alkylating agents within eight weeks before the screening visit or anticipated use during the time of the proposed study
• Subject is currently enrolled in or has not yet completed at least four weeks since ending other investigational device or drug trial(s), or subject is receiving investigational agent(s)
• Past or present participation in any study evaluating PEG-rHuMGDF, recombinant human thrombopoietin (rHuTPO), AMG 531, or related platelet product
• Pregnant (i.e. positive urine pregnancy test) or breast feeding
• Subject is not using adequate contraceptive precautions, if applicable.
• Known hypersensitivity to any recombinant E coli-derived product
• Subject has any kind of disorder that compromises the ability to comply with all study procedures