Twenty-six epidemics of asthma occurred in Barcelona between 1981 and 1987 in relation to inhalation of soy dust from grain silo unloading in the harbor. No further epidemics occurred after filters were installed, until a 1994 outbreak of asthma due to soy was detected in L'Hospitalet de Llobregat, a city on the outskirts of Barcelona. We describe 9 patients who presented with acute asthma attacks of sudden onset within a time span of 135 minutes. Counts of total IgE and IgE specific to soy extract were analyzed in all patients. Skin prick tests confirmed exposure to soy in all cases. Atmospheric levels of alcoholic fractions of soy particles (beta-sitosterol and stigmasterol) registered in L'Hospitalet de Llobregat were higher than those detected in Barcelona. We confirmed that soy had been unloaded in the harbor the same day, and that climatic conditions favored the movement of air from the port toward the city of L'Hospitalet. We postulate the need to periodically review preventive measures given the potential severity of the such epidemics.