Acute exogenous allergic alveolitis with the typical symptoms of unproductive cough, dyspnoea on exertion, fever, loss of weight, headache and limb pains was observed in a 24-year-old bank employee. Leucocytosis was accompanied by raised ESR; pronounced hypoxaemia and marked restrictive ventilatory defects were found. X-ray changes consisted of severely increased fine reticular interstitial shadowing. The differential diagnosis of allergic alveolitis was confirmed by the demonstration of precipitating antibodies in serum against Pullularia pullulans, Trichoderma viride, Cephalosporium acremonium and Aspergillus fumigatus, moulds with an ubiquitous occurrence. Typical changes of alveolitis were found histopathologically and immunohistologically in the lung tissue after a mini-thoracotomy. The source of exposition leading to the disease were pot plant earth and containers in the patient's flat. Moulds could be isolated from these substrates.