To elucidate the effect of the concomitant usage of other drugs on the occurrence of mucositis during and after chemotherapy, we investigated prescriptions for pediatric patients with hematological cancers at Kanazawa University Hospital, focusing particularly on drugs that have an adverse effect of xerostomia. The patients were divided into two groups consisting of methotrexate-treated (MTX group) and other-anticancer-drug-treated patients (non-MTX group). The occurrence of mucositis in the MTX group was significantly higher than in the non-MTX group. In the MTX-group, the number of concomitantly used drugs during a one-week period after chemotherapy was 6.18 in patients with mucositis and 2.95 in patients without mucositis. In the non-MTX group, there were 3.38 or 3.56 concomitantly used drugs in patients with or without mucositis, respectively. The concomitant drugs were divided into xerostomia-inducing and non-xerostomia-inducing drugs and there-after were classified into pharmacological groups. The occurrence of mucositis was higher among patients using xerostomia-inducing drugs than among those using non-xerostomia-inducing drugs with an exception of diuretics, and this tendency was more prominent in the MTX group than in the non-MTX group. It was recently documented that mucositis was significantly associated with xerostomia during 5-fluorouracil chemotherapy. Therefore, it is assumed that the concomitant usage of xerostomia-inducing drugs reduced the saliva flow, leading to mucositis. We conclude that the concomitant use of xerostomia-inducing drugs is one of the factors that influence the occurrence of mucositis during chemotherapy with MTX and other anticancer agents. It is recommended that the use of xerostomia-inducing drugs should thus be avoided as much as possible to decrease the occurrence of mucositis during chemotherapy.