The American Society of Clinical Oncology recently published a Clinical Practice Guideline entitled "Appropriate Chemotherapy Dosing for Obesity Adult Patients with Cancer." The panel recommended that full weight (actual weight)-based cytotoxic chemotherapy doses are used to treat obese patients with cancer, particularly when the goal of treatment is cure. However, no study has examined dosage calculation methods used for obese cancer patients in Japan. Here, we retrospectively studied the relationships between chemotherapy dose intensity, the occurrence of adverse events, and treatment outcomes in obese patients undergoing chemotherapy. Patients were divided into two groups: the actual BW group (BWg) was composed of patients receiving dosage amounts calculated using their actual BW (n = 64), and the ideal BWg was composed of patients receiving dosage amounts calculated using their ideal BW (n = 41). There were significant differences in the incidence of Grade 3/4 hematological toxicity in the actual and ideal BWg in solid tumor patients, but not in patients with hematological malignancies. In solid tumor patients with ≥30 body mass index (BMI), the incidence of Grade 3/4 hematological toxicity was significantly lower in the ideal BWg than in the actual BWg. Particularly, in patients with complications, incidence of Grade 4 hematological toxicity was significantly higher in the actual BWg than in the ideal BWg. These results suggest that the tumor type, degree of obesity, complications, and choice of chemotherapy regimen should be considered when determining chemotherapy dosage for obese patients.