The incidence of allergic alveolitis (AA) and of febrile reactions (FR) to inhaled mold dust in those who did not meet the criteria of AA was investigated in Swedish farmers, and the findings were related to farming methods and exposure type. A questionnaire was completed by 6,702 farmers. A stratified sample of 390 farmers underwent examination comprising spirometry, a skin-prick test, blood tests for precipitating antibodies and total IgE and an interview concerning symptoms, exposure and farming methods. The yearly incidence of AA was 2-3/10,000 farmers. The incidence of FR was 30-50 times higher (1/100 farmers) and the majority of these reactions was of a toxic rather than allergic type (organic dust toxic syndrome). Among farmers with recent FR or AA there was a higher frequency of positive precipitin reactions (p less than 0.05). Exposures to moldy grain, straw, wood chips and hay were associated with both AA and FR. A major difference in exposure between 17 farmers with AA and 80 with FR was that AA appeared to require repeated exposure, while FR was associated with occasional heavy exposure to mold dust.