['To determine the impact of prostate size on positive surgical margin (PSM) rates after robot-assisted radical prostatectomy (RARP) and the preoperative factors associated with PSM.', 'In all, 1229 men underwent RARP by a single surgeon, from 2005 to August of 2013. Excluded were patients who had transurethral resection of the prostate, neoadjuvant therapy, clinically advanced cancer, and the first 200 performed cases (to reduce the effect of learning curve). Included were 815 patients who were then divided into three prostate size groups: <31\u2009g (group 1), 31-45\u2009g (group 2), >45\u2009g (group 3). Multivariate analysis determined predictors of PSM and biochemical recurrence (BCR).', 'Console time and blood loss increased with increasing prostate size. There were more high-grade tumours in group 1 (group 1 vs group 2 and group 3, 33.9% vs 25.1% and 25.6%, P = 0.003 and P = 0.005). PSM rates were higher in prostates of <45\u2009g with preoperative PSA levels of >20\u2009ng/dL, Gleason score ≥7, T3 tumour, and ≥3 positive biopsy cores. In group 1, preoperative stage T3 [odds ratio (OR) 3.94, P = 0.020] and ≥3 positive biopsy cores (OR 2.52, P = 0.043) were predictive of PSM, while a PSA level of >20\u2009ng/dL predicted the occurrence of BCR (OR 5.34, P = 0.021). No preoperative factors predicted PSM or BCR for groups 2 and 3.'] Is number of positive preoperative biopsy cores a predictor of positive surgical margins ( PSM ) in small prostates after robot-assisted radical prostatectomy ( RARP )?