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I. The relative power of sixteen clinical, radiological and pulmonary function variables for evaluating asbestosis and chronic osbtructive airway disease has been assessed by principal component analysis of the data from a survey of 201 asbestos workers.2. A decrease in the transfer factor (diffusing capacity) for carbon monoxide followed by a decrease in the vital capacity had the greatest power to measure the severity of both types of disease but had little ability to distinguish between the two.3. In decreasing order of potency, the best indicators for distinguishing between asbestosis and obstructive airway disease were forced expiratory volume as a proportion of vital capacity, phlegm, radiological pleural thickening, cough and finger clubbing. Low values in FEV/VC and high values in phlegm and cough indicated obstructive airway disease: high values of pleural thickening and finger clubbing indicated asbestosis in the present context. 4. The analysis also provided numerical scores for each individual that could be plotted on a two-dimensional diagram. Orientation along one axis showed the degree of involvement of individuals by lung disease, whereas separation along the other axis depended on the nature of the disease process, asbestosis and obstructive airway disease in this instance.Although advanced asbestosis is readily recognized, there is wide disagreement about the order of appearance and relative importance of the initial manifestations.