We investigated the efficacy of clonazepam as an antiemetic in cisplatin-based chemotherapy for lung cancer. Seven patients experienced cisplatin-induced vomiting despite antiemetic therapy including 5-hydroxytryptamine(3) (5-HT(3)) antagonist and dexamethasone. Therefore, the antiemetic therapy including clonazepam, 5-HT(3) antagonist and dexamethasone was subsequently explored in the next course for the same seven patients. We administered clonazepam (0.5 or 1.0 mg/kg) once a day orally for 5 d from day one prior to chemotherapy. The grade of delayed vomiting, evaluated according to Common Terminology Criteria for Adverse Events Version 3.0, in the courses of therapy with clonazepam was significantly lower than without clonazepam (p=0.013). The patients whose serum clonazepam concentrations were below the lower limit of detection (3.0 ng/ml) experienced vomiting in three of three courses, whereas the patients whose serum clonazepam concentrations were higher than 4.3 ng/ml experienced no vomiting in six of seven courses. We observed that the symptom of cisplatin-induced delayed vomiting is controlled with serum clonazepam levels in the order of 10.0 ng/ml.