Twenty-eight persons were hospitalized in Illinois with neurologic signs and symptoms compatible with botulism in October 1983. Twelve patients required ventilatory support, and 20 patients were treated with trivalent ABE antitoxin; one patient died while still in the hospital six months after onset of illness. Type A toxin and/or type A Clostridium botulinum were subsequently identified in specimens from 18 patients. Case-control studies implicated sauteed onions made from fresh raw onions and served on a patty-melt sandwich in a local restaurant as the vehicle of transmission. Although the original sauteed onions were not available for toxin testing, type A toxin was detected in washings from a wrapper in which a patty-melt sandwich was taken home by one of the ill persons. Also, type A C botulinum was cultured from five of 75 raw onions taken from the restaurant. This outbreak implicated an unusual vehicle for botulinal toxin that was initially not suspected and demonstrates the importance of considering all theoretically possible food items as potential vehicles for toxin until epidemiologic and laboratory data have been collected and analyzed.