Summary Background Patients with incurable thoracic cancer often complain of a reduced ability to exercise, but the cause of this has been little studied. Thus, we have explored how various physiological and psychological factors relate to exercise performance in this group. Methods Inspiratory muscle strength, peripheral muscle power, lung function and mastery over breathlessness were assessed using sniff nasal inspiratory pressure, leg extensor power, simple spirometry and the mastery domain of the Chronic Respiratory Disease Questionnaire respectively. Exercise performance was assessed using the Incremental Shuttle Walking Test (ISWT) during which patients wore a K4 b 2 system permitting measurement of resting and breakpoint heart rate, minute ventilation (VE) and oxygen uptake (VO 2). Relationships between ISWT distance and the four factors were determined using correlation and β regression coefficients. Results Forty-one patients (21 male, mean (SD) age 64 (8) years) walked a median [IQR] of 320 [250–430] metres and reached a mean (SD) of 76 (10), 77 (25), and 48 (14) of their percent predicted maximum heart rate, VO 2, and VE respectively. Exercise performance was significantly associated only with inspiratory muscle strength ( r = 0.42, P < 0.01) and peripheral muscle power ( r = 0.39, P = 0.01). These factors were also significant determinants of exercise performance (β coefficients [95%CI] 1.77 [0.53, 3.01] and 1.22 [0.31, 2.14] respectively). Conclusion Of the factors examined, only inspiratory and peripheral muscle performance were significantly related to and predictive of exercise performance. Rehabilitation interventions which include inspiratory and peripheral muscle training are worth exploring further in this group of patients with thoracic cancer.