Romiplostim is recommended for the second- and third-line treatment of primary immune thrombocytopenia (ITP). We conducted a large, single-arm study (clinicaltrials.gov; NCT00508820) with broad entry criteria to evaluate the safety of romiplostim in adult ITP. Patients (n = 407) with ITP lasting 0.03-57.14 years and low platelet counts (median 14.0 × 109/l) or uncontrolled bleeding received romiplostim for up to 4 years. The rates of treatment-related, serious adverse events, serious hemorrhage events, thromboembolic events and fatal events were similar to those reported in previous romiplostim trials (0.2, 0.4, 0.2 and 0.1/100 patient-weeks, respectively). Bone marrow reticulin was observed in 4 patients, but biopsies were not routinely performed so the true incidence of this event cannot be determined. Type I collagen (nonserious, unrelated) was reported in 1 patient who likely had myelodysplastic syndrome. No new class of adverse events was reported. Platelet responses were achieved by >90% of the patients, typically within 1-2 weeks of the initiation of romiplostim treatment. From week 8, median platelet counts were >100 × 109/l; 47% of the patients received rescue medications (the use decreased over time). This study confirms and extends the tolerability/efficacy findings of previous romiplostim clinical studies. It was performed on a large ITP population, which is likely more representative of clinical practice.