From May 1 to December 26, 1981, a previously undescribed illness caused 277 deaths and the hospitalization of 12,656 persons in Spain. The disease was characterized by pneumonitis, eosinophilia, and marked loss of muscle mass and function. A succession of case-control studies was done in the town of Las Navas del Marqués (province of Avila) located 47 miles northeast of Madrid and with a population of 4009 people. The studies confirmed the association between illness and consumption of an illegally sold cooking oil. Other risk factors identified in the linear logistic regression analysis included involvement in various kitchen activities, increased average time spent inside the house, and the purchase of cooking oil from a specific street vendor. Although the oil was found to be a mixture of vegetable and animal fats contaminated with aniline and anilide-oil complexes, the specific etiologic mechanism of the disease has not been determined.