A method is described for converting the diagnostic codes of a disease registry from the eighth to the ninth revision of the International Classification of Diseases. The files of the Health Surveillance Registry contain records of nearly 100 000 cases with ICDA-8 coded disabilities. These data are used to determine incidence, prevalence, and other statistics relating to morbidity in the Province of British Columbia. The steps are outlined to convert the disability codes to ICD-9, and include the development of conversion tables, techniques for both computer and manual conversion of the codes for each case, and a description of the resources needed to complete the conversion within a set time-limit. The conversion methodology outlined here can be adapted to future revisions of disease classifications.