A 35 year-old nonatopic woman was referred to the hospital for possible work-related asthma. She had worked as an operator, at a plant producing polypropylene bags, for the previous four yrs. Her main complaint was a productive cough with dyspnoea and wheezing, as well as rhinitis over the past 3 yrs. She had been absent from work for 6 months on maternity leave, and had improved greatly. She was on a beta 2-adrenergic agent and had to take it at least four times daily. Baseline spirometry whilst at work showed marked airflow obstruction (forced expiratory volume in one second (FEV1) of 43% predicted (pred). After two months away from work FEV1 improved to 89% pred; provocative concentration of histamine causing a 25% fall in FEV1 (PC20) was 3.6 mg.ml-1 (mild airway hyperresponsiveness). Return to work resulted in a marked deterioration in FEV1, and serial peak expiratory flow (PEFR) values. PC20 was 0.11 mg.ml-1 (severe airway hyperresponsiveness) one week after she had returned to work. Specific inhalation challenges with polypropylene heated to 250 degrees C resulted in a late asthmatic reaction. As formaldehyde is one of the degradation products of heating polypropylene, we exposed her to it for up to 2 h, but we elicited no bronchospastic reaction. We conclude that heated polypropylene should be listed as one of the agents that causes occupational asthma.