The paper gives an account of the principal structure and main characteristics of the International Classification of Diseases, 1975 Revision (ICD-9), which has been used by Norwegian hospitals and the Norwegian public health administration since 1987. ICD-9 coding rules are examined, mainly the basic rule that transformation of diagnoses to codes should be complete, or should result in the smallest possible loss of information and convey no invented information. The rule of multiple coding states that, if possible, diagnoses which can be coded only with much loss of information should be divided into smaller entities. The ICD-9 code of the diagnosis comprises the codes of the set of entities which most reduces loss of information. If more than one set of entities represents the same amount of reduction in loss of information, then the set with the smallest number of entities should be chosen for reasons of reproducibility. The paper also presents the two procedures by which the corresponding ICD-9 category and code for a given diagnosis can be searched and identified.