The objective of this study was to examine the relationship between descriptors of breathlessness and its underlying cause in patients with lung cancer and cardiopulmonary diseases to see whether descriptors might be used to help determine the cause of breathlessness, particularly in patients with lung cancer. We studied 131 patients with primary or secondary lung cancer, whose breathlessness was attributed to tumor mass, pleural effusion, lung collapse, metastases, pleural thickening or lymphangitis carcinomatosis, and 130 patients with breathlessness attributed to asthma, chronic obstructive pulmonary disease (COPD), interstitial lung disease or cardiac failure. Patients selected statements (descriptors) that described the quality of their breathlessness from a 15-item questionnaire and the relationship between the descriptors and the attributed cause of breathlessness was evaluated by cluster analysis. All patient groups were characterized by more than one cluster and several clusters were shared between groups. Specific sets of clusters were associated with breathlessness due to asthma, COPD and cardiac failure, and to cancer causing collapse, metastases or pleural thickening. The association of different sets of clusters with the different diagnostic groups suggests that patients are describing qualitatively different experiences of breathlessness, but the relationship does not appear to be sufficiently robust for the questionnaire to aid differential diagnosis.