Postoperative hemithoracic irradiation is regarded as an important part of the curative treatment for resectable malignant pleural mesothelioma (MPM). Because the clinical target volume in postoperative MPM is irregular and surrounded by dose-limiting critical structures, intensity-modulated radiation therapy (IMRT) is thought to be suitable. However, postoperative hemithoracic IMRT remains experimental due to a high incidence of fatal pneumonitis. Therefore, a Phase I dose escalation study for hemithoracic IMRT using helical tomotherapy was planned, and the results of the first three patients are herein reported because this technique may provide benefits to such patients. For 3 patients with postoperative MPM, who were treated by extrapleural pneumonectomy (EPP), a radiation dose of 45.0 Gy in 25 fractions was given to cover 95% of the PTV. The lung V5s of the three patients were 14.3%, 10.0%, and 31.3%, respectively. The V5s of the present plans was smaller than that of the recent IMRT planning studies. The lung V20s of these patients were 2.4%, 2.2%, and 4.3%, respectively. Their MLDs were 4.3 Gy, 3.4 Gy, and 5.8 Gy, respectively. The follow-up periods of the patients were 26, 14 and 9 months from initiation of IMRT, respectively. All patients were alive, although local and contralateral recurrences had developed in 1 patient. Only 1 patient had Grade 2 acute esophagitis and nausea. There was no treatment-related pneumonitis. Hemithoracic IMRT using helical tomotherapy may play a crucial role in adjuvant treatment for MPM after EPP.