Background: Obesity adversely affects outcomes from adjuvant (adj) chemotherapy for breast cancer. Capping of chemotherapy dose has been implicated as one factor that may contribute to this. We postulated that if capping resulted in underdosing, this would be reflected in the incidence of dose dependent toxicities (DDT) such as mucositis and haematological toxicity. Methods: We retrospectively reviewed records of breast cancer patients (pts) commencing adj chemotherapy from 1 Jan 2010-31 Dec 2010 at the Auckland Regional Blood and Cancer Service identified using a prospective pharmacy database. Baseline characteristics, BSA, body mass index (BMI), neutrophil (ANC) nadir, relative dose intensity (RDI) and dose limiting complications (DLC) were recorded. Results: 174 pts were identified. Median age was 49 yrs (24-73). 17.8% had a BSA >2. 3.2% pts with BSA>2 had BMI <30. 42% had a BMI ≥30 (17.9-61.0), of whom 59% had a BSA ≤2. All patients with BSA >2 had dose capping. 71 pts had a DLC. Neutropenic fever/infection or treatment delays from neutropenia were more common if BSA ≤2 vs. >2 (p=0.08) but not with other DDT. Treatment deferrals (18.8 vs. 16.1%) dose reductions (25.2 vs. 32.3%) and mucositis were similar in both groups. Mean baseline ANC was 4.4 in pts with BSA ≤2 and >2. Mean ANC nadir was 0.8 and 1.3 respectively (p<0.01). r²BSA vs nadir ANC was 0.012. G3/4 neutropenia was seen 76.5% of pts with BSA ≤2 vs. 44.8% if BSA >2 (RR 1.3 p<0.01). No pt had > G1 thrombocytopenia. Mean RDI was 93.2 and 94.8 respectively (p=0.4) Conclusions: Previous studies show ANC nadirs correlate well with drug exposure (area under plasma concentration vs. time curve) but poorly with BSA. Therefore we used hematological and DDT as surrogate measures for drug exposure in pts with and without capped doses. There was more G3/4 neutropenia (p<0.01) in uncapped pts but rates of other DDT were similar. Since BSA based dosing has little impact on inter-pt variability in systemic drug exposure and trials where capping was not used still show a poorer outcome in obese pts, these results suggest dose capping is a minor contributor to the worse outcomes in morbidly obese breast cancer pts having adj chemotherapy. Addressing obesity itself may be more important.