A 52 year old farmer was referred to us for investigation of suspected farmer's lung. For many years the farmer had been exposed to hay, straw, pigeons, and fuel chip dust. Under exertion he suffered from shortness of breath. In the farmer's own fuel chips we could identify Aspergillus fumigatus, Paecilomyces species and Mucor species. In the farmer's blood we found IgG-antibodies against his own fuel chips, thermophilic actinomycetes, Penicillium species, Mucor species and Aspergillus fumigatus. We did not detect any IgG-antibodies against pigeon serum or pigeon faeces. In order to determine the responsible allergen we performed two challenge tests. In the first test the farmer had to inhale his own hay and straw dust for one hour. This provocation was negative. A second one-hour inhalative challenge was carried out 16 days later using his own fuel chips. This time he experienced significant pulmonary and systemic reactions: body temperature rose by 3.3 degrees C, leucocytes by 12,200/mm3; PO2 fell by 39.4 mmHg, vital capacity by 52%, DLCO by 36%. After the challenge the farmer complained of coughing and dyspnoea. Rales could be heard on auscultation, and an interstitial infiltrate was seen to develop on chest x-rays. After the challenge the patient had to be treated with oxygen and systemic corticosteroids. We diagnosed a fuel chip-induced exogenous allergic alveolitis (EAA). Eight days later the parameters were back to normal and the farmer was discharged from our hospital with further corticosteroid medication. This method of inhalative provocation is very important in diagnosing an EAA. Problems arise when the mode and duration of exposure to substances has to be chosen. Because of the risk of severe reactions, inhalative provocations relating to EAAs should only be performed in special centres with an intensive care unit. In this paper we present a diagnosis of fuel chip lung, which is rarely seen in Germany. However, with the rising use of fuel chips as heating material it is necessary to consider this use as a cause of EAA among farmers.