The rationale for this Phase III study is to evaluate the 6 month safety and efficacy of eltrombopag in the treatment of previously treated subjects with chronic ITP. The starting dose of eltrombopag, 50 mg, once daily was selected based upon the observed efficacy, safety and pharmacokinetics in a dose-finding Study (TRA100773). This Phase III study is a randomized, double-blind, placebo-controlled, Phase III study, to evaluate efficacy, safety and tolerability of eltrombopag, initially administered as 50 mg oral tablets once daily for six months in adult subjects with previously treated chronic ITP. Subjects will be randomized 2:1, eltrombopag to placebo, and will be stratified based upon splenectomy status, use of ITP medication at baseline and baseline platelet count less than or equal to 15,000/µL. Subjects will receive study medication for 6 months, during which the dose of study medication may be adjusted based upon individual platelet counts. In addition, subjects may taper off concomitant ITP medications and may receive any rescue treatments as dictated by local standard of care. After discontinuation of study medication, subjects will complete follow-up visits at weeks 1, 2, 4 and months 3 and 6.