In the investigation of dyspnoea and severe hypoxaemia the clinical relevance of multiple diagnostic techniques was studied. The patient was sequentially studied utilizing several techniques. The degree of lung impairment by spirometry, diffusing capacity for carbon monoxide, haemodynamics, pulmonary gas exchange, ventilation-perfusion relationships assessed by the multiple inert gases elimination techniques, ventilation and perfusion lung scans, gallium 67 scintigraphy, bronchoalveolar lavage and high resolution computerized tomography, twice over a period of 12 months during recovery under treatment. A marked impairment of pulmonary gas exchange was first explained by diffusion impairment and ventilation-perfusion mismatch. The multiple inert gas elimination technique allowed determination of the cause of hypoxaemia by ventilation-perfusion inequality. A pathological correlate of the ventilation-perfusion inequality was the appearance of honeycomb lungs detected by high resolution computed tomograph and active alveolitis by bronchoalveolar lavage. All results were consistent with a diagnosis of fibrosing alveolitis. The patient was evaluated again during treatment. Some functional improvement occurred despite persistence of the same pathological findings. In conclusion, this study demonstrates the value of information derived from different tests. Physiological correlations complemented by pathological observations expand understanding of the pathogenesis of disease. These procedures contribute to understanding mechanisms responsible for functional impairment.