In this paper standardized and quantitative definitions of perioperative risk and risk factor using probabilities are given. A calculation of risk and risk factors is performed using data from a study on perioperative risk in colon resection and a study on a preoperative risk check in general surgery. The problem of one risk factor, a combination of two risk factors and the use of many risk factors to quantify preoperative risk is discussed. Confidence intervals are recommended as a standard method for presenting statistical results on perioperative risk.