While some of the gaseous and particulate components of diesel exhaust can cause pulmonary irritation and bronchial hyperreactivity, diesel exhaust exposure has not been shown to cause asthma. Three railroad workers developed asthma following excessive exposure to locomotive emissions while riding immediately behind the lead engines of caboose-less trains. Asthma diagnosis was based on symptoms, pulmonary function tests, and measurement of airways hyperreactivity to methacholine or exercise. One individual's peak expiratory flow rates fell in a work-related pattern when riding immediately behind the lead diesel engine. None had a previous history of asthma or other respiratory disease and none were current smokers. All three developed persistent asthma. In two cases, physiologic abnormalities suggesting reversible restriction were observed. This is the first report implicating diesel exhaust as a cause of reactive airways disease.