The effect of percutaneous kidney biopsy on glomerular filtration rate has never been identified, though it is frequently a concern raised by patients. Following a clinical interaction with an inquisitive patient undergoing her fifth biopsy, we attempted to estimate the effect using retrospective data. In a cohort of patients with stable kidney function undergoing transplant biopsy without clinical indication (as part of a surveillance programme) the effect of biopsy was observed as a step change in glomerular filtration rate. Reassuringly, the loss of glomerular filtration rate resulting from a biopsy, has a 1-sided 95% confidence interval of <1.4 mL/min.