The RACS sentinel node biopsy versus axillary clearance (SNAC) trial compared sentinel-node-based management (SNBM) and axillary lymph-node dissection (ALND) for breast cancer. In this sub study, we sought to determine whether patient ratings of arm swelling, symptoms, function and disability or clinicians' measurements were most efficient at detecting differences between randomized groups, and therefore, which of these outcome measures would minimise the required sample sizes in future clinical trials. 324 women randomised to SNBM and 319 randomised to ALND were included. The primary endpoint of the trial was percentage increase in arm volume calculated from clinicians' measurements of arm circumference at 10 cm intervals. Secondary endpoints included reductions in range of motion and sensation (both measured by clinicians); and, patients' ratings of arm swelling, symptoms and quality of life, using the European Organisation for Research and Treatment of Cancer Breast Cancer Module (EORTC QLM-BR23), the body image after breast cancer questionnaire (BIBC) and the SNAC study specific scales (SSSS). The relative efficiency (RE, the squared ratio of the test statistics, with 95% confidence intervals calculated by bootstrapping) was used to compare these measures in detecting differences between the treatment groups. Patients' self-ratings of arm swelling were generally more efficient than clinicians' measurements of arm volume in detecting differences between treatment groups. The SSSS arm symptoms scale was the most efficient (RE = 7.1) The entire SSSS was slightly less so (RE = 4.6). Patients' ratings on single items were 3-5 times more efficient than clinicians' measurements. Primary endpoints based on patient-rated outcome measures could reduce the required sample size in future surgical trials.