Background. The management of chemotherapy-induced nausea and vomiting (CINV) is of primary concern for both patients with cancer and medical workers. Refractory or breakthrough CINV is especially difficult to deal with and necessitates a different approach. Vitamin B1 deficiency is likely to occur during cancer chemotherapy, with early symptoms of fatigue, anorexia, nausea and vomiting. The efficacy of vitamin B1 for the treatment of delayed or refractory CINV should be confirmed. Methods. Serum vitamin B1 level was prospectively measured in patients experiencing persistent nausea and vomiting after chemotherapy. The response to vitamin B1 therapy was evaluated for three consecutive days after vitamin B1 infusion. Moreover, serum level of vitamin B1 at diagnosis of persistent delayed CINV was compared with the level before chemotherapy. Results. In total, 408 courses of chemotherapy in 86 patients were analyzed. The median age at hospital admission of the enrolled patients was 10.7 years (0.2–25.2). Among these, 44 (10.8%) episodes of persistent delayed CINV were identified in 26 of the enrolled patients. At day 3, the overall response rate was 79.5%; 21 (47.7%) patients achieved a complete response and 14 (31.8%) patients achieved a partial response. The median vitamin B1 level at diagnosis of CINV was significantly lower than the value before chemotherapy (22.8, range 11.9–49.2 vs. 32.7, range 11.2−80.1, respectively, P < 0.001). Conclusion. Patients with a malignant disease who experience persistent nausea and vomiting after chemotherapy often exhibit vitamin B1 deficiency. Vitamin B1 infusion may be beneficial for many of these patients.