Autocategorization is a new technique appropriate for studies involving relative frequencies of illness diagnoses using the International Classification of Diseases. It is ideally suited for emergency department illness patterns. The technique uses the standard chi-square statistical test, but is novel in objectively determining the aggregation of diseases into categories for analysis. The numbers of observed occurrences of the various diseases are used. without applying any a priori judgements, to decide whether a disease should be analyzed alone, or aggregated with others. This technique, because it is based on the ICDA code, will permit comparisons between various studies while retaining the statistical validity of the individual studies.