Clinical symptoms and exposure conditions were investigated in 80 farmers with organic dust toxic syndrome, defined as the occurrence of febrile reactions after exposure to organic dust in subjects with no evidence of allergic alveolitis. The material was compiled from a field study of febrile reactions in the farming community and the diagnosis was based on interviews performed by physicians. Of the 75 men (mean age 44) and five women (mean age 39), only 13% of the men and none of the women were current smokers. One attack had been experienced by 44% and the remaining subjects had had two or more attacks, often several years apart. The duration of symptoms was 24 hours or less in 46% of the farmers and in 95% of the cases the symptoms lasted less than one week. The attacks were most common in the autumn and were usually provoked by handling grain (80% of the farmers with organic dust toxic syndrome). Other causes were hay, straw, wood chips, and silocapping material. The material was usually described as extremely mouldy and the episodes were usually provoked by unusual work tasks such as cleaning grain bins or removing mouldy feed. Twenty three farmers had consulted physicians: five of nine examined during symptoms had slightly abnormal chest radiographs and two of four examined had decreased arterial oxygen tension. Spirometry performed during a symptom free interval was normal.