Audits of adjuvant chemotherapy for breast cancer have revealed that obese patients receive a lower relative dose intensity (RDI). However, interpretation of these studies is complicated by the variable use of cytokine growth factors, empiric dose capping and first cycle dose reductions. We have analysed the impact of obesity on RDI in a cohort of 662 patients that is not confounded by these factors. Patients were classified as overweight or obese on the basis of a body mass index (BMI)>or=25 kg/m2. The mean RDI in patients with BMI>or=25 kg/m2 was actually significantly greater than in those with BMI<25 kg/m2 (p=0.03). Overweight patients were less likely to experience cycle delays due to prolonged myelosuppression (p<0.001), particularly towards the end of the treatment course. We conclude that drug doses need not be reduced on the basis of obesity. Overall obese patients are in fact less likely to suffer haematological toxicity.