We present results of the use of a computerized system for the automated interpretation of standard laboratory measures of pulmonary function and arterial blood gases. This system is now in routine use in Presbyterian Hospital, Pacific Medical Center. The program produces a report, intended for patient records, that explains the clinical significance of measured quantitative test results and gives a diagnosis of the presence and severity of pulmonary disease in terms of the measured data, referral diagnosis, and patient history. “Rules” are used by both the physiologist and the computer system to specify the system operation. “Rules” are statements of the form “IF [condition] THEN [conclusion]”, ie “if a set of facts are true” then “make a conclusion based on these facts”. The sequence of rules used to interpret the case also specifies a line of reasoning about the case. The rules also contain information usable for making a detailed explanation of the interpretation of the case. The use of rules for this type of knowledge based system is taken from the results of applied Artificial Intelligence research. In a 144 case prospective evaluation, there was a 91% overall rate of agreement between the rule based system diagnoses and the diagnoses of the designing physiologist; there was a 89% rate of agreement between the system diagnoses and diagnoses of a second independent physiologist.