The efficacy and safety of romiplostim, a thrombopoietin-mimetic peptibody, were evaluated in a double-blind, placebo-controlled, randomized trial of Japanese patients with chronic immune thrombocytopenia (ITP). Thirty-four ITP patients received romiplostim (n = 22) or placebo (n = 12) for 12 weeks, with a starting romiplostim dose of 3 μg/kg weekly. The primary end point was the number of weeks with platelet response, defined as a platelet count ≥50 × 10⁹/L (not including the 4 weeks after rescue medication administration). Patients received a median of 4 (range 1–19) prior ITP therapies including splenectomy in 44%. On study, 68% also received concomitant ITP therapy. Weekly responses occurred for a median of 11 weeks with romiplostim as compared to 0 weeks with placebo (p < 0.0001). Most romiplostim-treated patients (95%) achieved platelet responses; two showed extended responses after the treatment period. The use of rescue medication was required in 9% of romiplostim-treated patients as compared with 17% of placebo-treated patients. Both treatment groups had similar incidences of adverse events (91% romiplostim, 92% placebo). Adverse events that occurred more frequently (>10%) in romiplostim-treated patients included nasopharyngitis, headache, peripheral edema, back pain, and extremity pain. In conclusion, romiplostim significantly increased and maintained platelet counts and was well tolerated in Japanese patients with ITP.