A diagnostic examination may be called rational if it leads to a short, less costly and especially more certain diagnosis for the patient. Among other things, sensitivity and specificity, i.e. the proportion of correct positive to correct negative results of the separates steps of the examination determine the reliability of the answers to diagnostic questions. In most hypertensive patients the diagnosis can be completed in medical general practise. First results in West Germany show that only about 5 to 10% of patients require further investigations in special practices, general hospitals or special departments.