Reported asthma morbidity and mortality are increasing in the U.S. We addressed one explanation, that the accuracy of the diagnosis of asthma is changing. The diagnosis of asthma was evaluated in 320 inpatient and outpatient records bearing the diagnosis of asthma for the periods 1970-73 and 1980-83 in a health maintenance organization (HMO). We determined whether or not our agreement with the chart diagnosis was a function of: sex, period of treatment, inpatient vs outpatient setting, whether or not asthma was the primary or secondary diagnosis, and patient age. The standard of comparison was an expert panel review in which asthma was divided into six categories. In both inpatient (97%) and outpatient settings (94%), the majority of charts examined exhibited a clinical picture consistent with asthma. The rate of the narrowly defined "definite asthma" category varied with respect to age, with the highest proportion in the under 20-year age group (74%) and the lowest (46%) in the over 60 age groups, probably because older individuals often have coexisting smoking related diseases. The increase in "definite asthma" among outpatients from the 1970s to the 1980s likely reflects increasing chart documentation among physicians, illustrating the need for clear, consistent chart documentation of signs and symptoms of asthma.