Seventeen victims of farmer's lung were monitored during two indoor feeding seasons for cattle. The victims, who had all recovered from the acute phase of the disease before entering the experiment, used powered helmets for respiratory protection (Air-Stream) equipped with Type P2 filters. Filters were changed once a week. The lung function of each subject was investigated at the beginning and towards the end of the indoor feeding season. Each subject kept a diary about the use of dust respirators, changing of filters, inconvenience related to the use of respirators, and symptoms experienced during the follow-up. None of the subjects reported symptoms they related to farmer's lung, and mean values for the lung function parameters (PEF, FEV1, FVC, DLCO, and KCO) did not decrease during the two follow-up periods. In contrast, mean DLCO and KCO increased slightly. At the very end of the both indoor feeding seasons, after the actual follow-up periods, one subject developed mild recurrences of the disease without deterioration detectable by radiology. The patient asserted that he had used the protective helmet properly. The results indicate that powered respirator helmets have protective value in farmer's lung and are appropriate for long-term use. A possibility remains, however, that the disease may recur in highly sensitized individuals, despite the use of efficient protective devices. Other measures for preventing or decreasing mould exposure are equally important.