This protocol will provide open label romiplostim to adult thrombocytopenic subjects. Romiplostim will be administered by subcutaneous injection once per week. Dose adjustment will be based on platelet counts, and will be allowed throughout the duration of the study. Rescue therapies are allowed at any time during the study. Reductions in concurrent ITP therapies may occur at any time when platelet counts are > 50,000.

Inclusion Criteria:

• Subject is ≥ 18 years of age
• Subject has a diagnosis of immune (idiopathic) thrombocytopenic purpura per the American Society of Hematology guidelines
• If Subject is > 60 years of age, subject has a written bone marrow aspiration and/or biopsy report consistent with a diagnosis of ITP
• Subject has received at least 1 prior therapy for ITP
• Subject's platelet count is ≤ 30,000 or the subject is experiencing bleeding that is uncontrolled with conventional therapies
• Subject (or legally-acceptable representative) is willing and able to provide written informed consent

Exclusion Criteria:

• Subject has a history of hematological malignancy, myeloproliferative disorder, myelodysplastic syndrome (MDS), or bone marrow stem cell disorder
• Subject has participated in any study evaluating PEG-rHuMGDF, recombinant human thrombopoietin (rHuTPO), or related platelet product
• Subject has a known hypersensitivity to any recombinant E coli-derived product
• Subject has received any therapeutic drug or device that is not approved by the local regulatory health agency for any indication within 4 weeks of Screening
• Subject is of reproductive potential and is not using adequate contraceptive precautions, in the judgment of the investigator
• Subject is pregnant or breast feeding
• Investigator has concerns regarding the subject's ability to comply with the protocol procedures

Average duration: cohort 1 52 wks; cohort 2 42 wks.

The table of Other Adverse Events summarizes the most frequent non-serious occurrences of adverse events.