A system has been developed for the interpretation of laboratory findings which can aid in medical diagnosis as well as monitoring medical treatment. Tools are provided for defining, updating and verifying rules for the interpretation of laboratory tests in a computer-acceptable form. The present system uses a set of arithmetic logic rules which allows comprehensive evaluation of laboratory data as well as bringing into focus fluctuating and borderline situations and inconclusive data. Application of the system for over a year has shown that it is easy to use by laboratory staff with no knowledge of programming. Moreover, it has forced greater comprehensiveness, objectivity, consistency and speed in the interpretation of laboratory results than is generally achieved by standard practice. © 1978.