Fifty patients with farmer's lung disease (FL), 20 asymptomatic exposed farmers, and 18 healthy controls were investigated for total IgG and total IgA, as well as subclass antibody levels against Aspergillus umbrosus, the most common fungus to which FL patients in Finland have been exposed. Enzyme-linked immunosorbent assay (ELISA) revealed significantly elevated levels of specific total IgG and IgA antibodies in the sera of patients during the acute phase of the disease. Both IgG1 and IgG2 subclass antibodies and both IgA1 and IgA2 subclass antibodies were significantly associated with the disease. The antibody levels decreased during the first 3 months, when mould exposure was avoided and when an oral 6-week steroid treatment was administered. During the next 12 month follow-up, the mean antibody levels continued to decrease, even though re-exposure might have occurred. The results (with sensitivity 94% and specificity 95%) imply that in addition to the measurement of A. umbrosus-specific IgG, determination of both A. umbrosus-specific IgG1 and IgA might also be useful in the scrodiagnosis of acute stage of FL.