A 34 year old sawmill maintenance engineer developed a dry cough that was associated with widespread wheezes and crackles in his lungs. His symptoms worsened, with work related lethargy, fever, and breathlessness, and the loss of a stone in weight. At that time, while still at work, he had a neutrophil leucocytosis and increased concentration of gamma globulins. When seen subsequently some two months after stopping work, his chest radiograph and lung function tests were normal, but the cells recovered at bronchoalveolar lavage showed an increase in lymphocytes and mast cells, a pattern consistent with extrinsic allergic alveolitis. Serum precipitins were identified to extracts of sawdust, wood chips, and bark from the sawmill, and to eight species of mould grown from these samples. Specific IgG binding inhibition studies suggested that a common epitope present on Trichoderma koningii might be responsible for the cross reactivity of the patient's serum with the wood and fungal extracts. A diagnosis of wood associated extrinsic allergic alveolitis was made and since changing his job the patient has remained well. Wood associated allergic alveolitis has not previously been described in British sawmill workers, but has been reported in Sweden, with a prevalence of 5%-10% in exposed workers. A review of published data suggests extrinsic allergic alveolitis in wood workers is primarily caused by inhalation of the spores of contaminating fungi, but inhaled wood dust may exert a synergistic effect.