Drinking red wine may provoke bronchospasm in subjects with asthma. In order to reveal some of the possible agents involved in this reaction, 18 patients with a history of red wine-induced asthma were studied. They received, in a double-blind fashion, red wine with low sulfur dioxide (SO2) and high amine, high SO2 and high amine and low SO2 and low amine content. In each challenge, the wine was administered in stepwise increasing quantities until a total of 385 ml or a fall in peak expiratory flow of greater than 15% was reached. Nine subjects demonstrated a significant fall in peak flow in one or more challenges. In all cases the most severe reaction was observed after the wine with high SO2 content. The study suggests that SO2 is the most important factor in red wine-induced asthma. It is recommended that wine labels provide information on the SO2 content.