An explosive outbreak of a febrile respiratory illness occurred among members of a college fraternity. The preponderant signs and symptoms were muscle aches, cough, and low-grade fever. All illnesses occurred within 1.3 to 13 hours of attendance at a party where there was a dense airborne dust from straw that had been laid on the floor. Of the 67 fraternity members who attended the party and answered a questionnaire, 55 became ill (attack rate, 82%). Risk of illness was higher for those who spent more time at the party. Duration of illness ranged from 4.5 hours to seven days. Results of serological studies did not demonstrate an allergic or viral cause for these illnesses. The clinical and epidemiologic features of this outbreak were characteristic of organic dust toxic syndrome, an acute respiratory illness caused by inhalation of molds growing on hay, silage, or other agricultural products.