The recovery of pulmonary function in farmer's lung (FL) was studied during a 5-yr follow-up in 101 patients (20 men and 81 women, mean age 47 yr). At the initial evaluation all patients had the first diagnosed attack of FL. Spirometry, measurement of pulmonary diffusing capacity, and arterial blood gas analysis were done at the time of the diagnosis and 1, 3, 6, 12, and 24 months thereafter. The last follow-up measurements were made 4 to 6 yr after the initial evaluation. Mean FVC improved significantly for up to 1 yr. In mean DLCO there was improvement for up to 2 yr. Mean PaO2 rose to its maximum within 1 month of the initial examination and did not change significantly thereafter. No patient had diagnosed asthma at the time of the diagnosis of FL, but asthma was later diagnosed in 7 patients during the follow-up. In conclusion, the pulmonary function of FL patients improved for up to 2 yr after the initial acute episode. In general, PaO2 improved most rapidly; the improvement in FVC was slower, and the recovery of DLCO took the longest time.