To rationally discuss the "need for preoperative laboratory investigations prior to performing an operation that can be pre-planned", two questions must be considered: 1. What kind of complications were encountered during or after 25,872 anesthesias performed in a 20-year period? 2. What kind of diseases were recognized only postoperatively which apparently could not be diagnosed preoperatively via case history and clinical examination? The results of these analyses are tabulated and assessed. It could be shown that the case history and the clinical examination of the patient on the day of surgery rank before all laboratory tests. With regard to peroperative respiratory disturbances it could be proven that children within the first trimester are particularly susceptible to such alterations. Furthermore, it could be demonstrated that within this group premature babies are more susceptible to this kind of complication than babies born at date. With regard to the incidence of preoperative respiratory disturbances in children with or without infection of the upper respiratory tract, it was shown that children without such symptoms who had however undergone an infection three to four weeks before the operation, were more susceptible to peroperative respiratory disturbances than children without any infection or even children suffering from an acute infection. Taking the physiological Hb decrease at the end of the first trimester as an example, questions on the optimal date for surgery are discussed. Preoperative handling is discussed on the basis of the blood glucose level of 500 children after a fasting period of 3-4 hours.