We studied the prevalence of asthma in a population of the South Fore linguistic group of the Eastern Highlands Province of Papua New Guinea, where a remarkable increase in the disease has been reported in the last decade. To make the study as objective as possible it was necessary to define the disease and to develop tests of bronchial reactivity for use in the field. Asthma was defined as symptoms of intermittent breathlessness plus bronchial hyperreactivity measured by a histamine inhalation test (HIT) or response to bronchodilator aerosol. Following a demographic study to define the symptomatic people in the population, all people with present or past breathlessness had a detailed history taken, their bronchial response measured with a HIT or with bronchodilator aerosol, and skin prick tests performed. Asthma was estimated to be present in at least 7.3% of the adult population and in 0.6% of children. The exact prevalence could not be determined because of difficulties in distinguishing asthma from chronic obstructive lung disease in patients with severe airflow limitation. In most subjects the asthma was severe and associated with allergy to house-dust mite. Control subjects matched for age, sex and village, who had no symptoms, had normal bronchial reactivity. The HIT, together with a history of breathlessness, may prove to be the simplest and most objective method for determining the prevalence of asthma in different populations.