Inclusion Criteria:

• Subject or subject's legally acceptable representative has provided informed consent.
• Subject is less than 18 years old at time of screening.
• Subject completed a romiplostim study for the treatment of thrombocytopenia in pediatric subjects with ITP.

Exclusion Criteria:

• Subject has or previously had any 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).
• Subject has any new active malignancy diagnosed since enrollment in the previous romiplostim ITP study.
• Subject received any alkylating agents within four weeks before the screening visit or anticipated use during the time of the proposed study.
• Other investigational medications are excluded.
• Currently enrolled in another investigational device or drug study, or less than 30 days since ending another investigational device or drug study(s), or receiving other investigational agent(s).
• Subject (male or female) is not willing to use highly effective contraception during treatment if and when sexually active and for 4 weeks (women) or 14 weeks (men) after the end of treatment.
• Subject is pregnant or breast feeding, or planning to become pregnant within 4 weeks after the end of treatment.
• Male subject with a pregnant partner who is not willing to use a condom if and when sexually active during treatment and for 14 weeks after the end of treatment.
• Subject has known sensitivity to any of the products to be administered during dosing.
• Subject previously has entered this study (this will depend on the type of study).
• Subject will not be available for protocol required study visits, to the best of the subject and investigator's knowledge.
• Subject has any kind of disorder that, in the opinion of the investigator, may compromise the ability of the subject to give written informed consent and/or to comply with all required study procedures.