OBJECTIVE: To determine the efficacy of the thrombopoietin receptor agonist romiplostim for the prevention of temozolomide-induced thrombocytopenia in newly diagnosed glioblastoma. METHODS: In the PLATUM phase II open-label, multicenter, single-arm trial, patients diagnosed with Common Terminology Criteria for Adverse Events grade 3 or 4 thrombocytopenia during chemoradiotherapy received weekly subcutaneous romiplostim injections. PLATUM aimed at demonstrating that the percentage of thrombocytopenic patients treated with romiplostim able to complete 6 cycles of maintenance temozolomide chemotherapy exceeded 10% (pp RESULTS: Twenty patients were enrolled in step 1. Median age was 61 years (range 33-73). Twelve patients received 6 temozolomide cycles, corresponding to a success rate of 60% (95% confidence interval 36%-81%). Four patients discontinued temozolomide because they did not respond to romiplostim, 2 for progression, and 2 for adverse events unrelated to romiplostim. CONCLUSIONS: The thrombopoietin receptor agonist romiplostim improves exposure to chemotherapy in patients with glioblastoma experiencing temozolomide-induced thrombocytopenia. NCT02227576. METHODS: This study provides Class IV evidence that for patients with glioblastoma and thrombocytopenia, romiplostim is effective for the secondary prophylaxis of temozolomide-induced thrombocytopenia.