Background: Chemotherapy (ctx) treated breast cancer patients (BC pts) w/ BMI ≥ 35 (WHO Class II obese) have significantly lower overall survival (OS) than pts w/ BMI < 35, regardless of race or age, possibly due to use of a lower dosing BSA (dBSA), to avoid ctx toxicity. This retrospective analysis examines discrepancies between calculated BSA (cBSA) & dBSA in BC pts. We further analyzed OS in relation to BMI & BSA discrepancies. Methods: Data obtained from the GHSU tumor registry included BMI, dBSA, cBSA (using Mosteller eq.), & OS data from all BC pts receiving ctx between 1997-2005. For pts w/ BMI ≥ 35, (n = 68) & pts who have BMI < 35 (n = 193), Lin’s coefficient of concordance (r) was used to measure agreement between the BSAs. Results: In pts w/ BMI < 35, agreement between the BSAs was almost perfect (r = 0.95, 95% CI 0.93-0.96). Yet, for pts w/ BMI ≥ 35, agreement between the BSAs was low (r = 0.55, 95% C.I. 0.43-0.66). BSA concordance differed significantly between the two groups (p < 0.001). Bias in BSA measurement was estimated by subtracting the cBSA from the dBSA for each pt. For all pts (n = 261), the mean bias ± SD = -0.06 ± 0.09 (range: -0.51 to 0.45). The mean bias differed significantly between pts w/ BMI > 35 (mean ± SD: -0.15 ± 0.12) & pts w/ BMI < 35, (mean ± SD: -0.027 ± 0.047), p < 0.001. In deceased pts (n = 87), the mean bias ± SD was -0.069 ± 0.078 (range: -0.4 to 0.03). In living pts (n = 174), the mean bias ± SD was -0.055 ± 0.096 (range: -0.51 to 0.45). Bias difference between those who died & those alive was not statistically significant (p = 0.2). Vital status differences are small between groups using odds ratio (OR) adjusted for the bias in BSA. Unadjusted comparison based on logistic regression: OR = 2.05, p = 0.014; adjusted for BSA bias: OR = 2.25, p = 0.028. Results are similar when comparing groups in terms of OS using hazard ratio (HR) adjusted for bias in BSA. Unadjusted comparison using Cox regression: HR = 1.56, p = 0.049; adjusted for BSA bias; HR = 1.65, p = 0.04. Conclusions: Our analyses indicate that there is a significant bias in BSA calculation for ctx dosing in pts w/ BMI ≥ 35. Yet, this bias does not seem to affect OS between BMI groups. The decreased OS seen in pts w/ BMI ≥ 35 is likely related to factors other than lower dBSA.