An epidemic of a new illness involving multiple organ systems began in Spain in May 1981, with 19,828 cases and 315 deaths reported by June 1, 1982. An epidemiologic investigation has linked the occurrence of illness with ingestion of an unlabeled, illegally marketed cooking oil. To elucidate the natural history of this illness, we reviewed the medical records of 121 patients in one severely affected town 100 km northwest of Madrid. The findings during the first week after onset were those of a febrile, pneumonia-like illness. Gastrointestinal findings and striking eosinophilia became prominent later in the first month. Although the disease followed a self-limited course in many patients, severe neuromuscular manifestations (myalgia severe enough to restrict movement, motor deficits, atrophy of major muscle groups, and contractures of the jaw and extremities) occurred late in the course of the illness (an average of 96 days after onset) in 23 per cent of the patients. The onset of disease early in the epidemic and particularly severe initial systemic findings were associated with progression to neuromuscular illness.