BACKGROUND Many changes in the morbidity of patients attended to the Country's hospitals have occurred in recent years, wether resulting from demographic, epidemiological, financial and technological factors or from those related to the structure of the services themselves. METHODS The change in diagnoses occurring over this period was analyzed using two data banks relating to hospital discharges during 1975 and 1988, in the Vale do Paraíba, Southeastern Brazil, taking into account the factors mentioned above. Both data banks covered the same variables: hospital, dwelling, patient's age and sex, health status at discharge, financing sources, duration of stay and main diagnosis. RESULTS Among the results were to be found: an ageing of the population of the region, with an increase in life expectancy and a decrease in the general and infant mortality coefficients, with a proportional diminution of mortality from "infectious and parasitic diseases" and an increase in "neoplasms", "diseases of the circulatory system" and "injury and poisoning". There was a drop in the number of hospitalizations by some diagnostic groups such as "symptoms, signs and ill-defined conditions" and "infectious and parasitic diseases" and an increase in others, particularly the "neoplasms" and "diseases of the circulatory system". There was a shift from the poorly-defined to better defined diagnoses, further, there was a decrease in the duration of stay for all groups. It was possible to identify some factors which might have caused these changes, although there was no one behavioral pattern for all diagnoses.