Dyspnea is a devastating symptom in patients with advanced cancer. Unfortunately, very limited research has been done on the frequency and correlates of dyspnea in this particular patient population. The purpose of this prospective study was to assess the frequency of moderate to severe dyspnea and the correlates of dyspnea in a population of ambulatory terminally ill cancer patients. One hundred thirty-five consecutive patients attending a multidisciplinary pain clinic were tested for respiratory function (vital capacity, peak flow, maximal inspiratory pressure, and oxygen saturation). All patients gave their rating of dyspnea, anxiety, and fatigue/tiredness using visual analogue scales (VAS). Lung involvement by the tumor (primary or metastatic) was determined from the patient's chart. Moderate dyspnea occurred in 74/135 (55%) patients with terminal cancer. Lung involvement (r = 0.285, P = 0. 0009), anxiety (r = 0.306, P = 0.0003), fatigue/tiredness (r = 0.211, P = 0.0146), and vital capacity (r = -0.189, P = 0.0444) were significantly correlated with the intensity of dyspnea. Multivariate analysis demonstrated that lung involvement (P = 0.0016) and anxiety (P = 0.0027) were independently correlated with the intensity of dyspnea. In the subgroup of patients with moderate to severe dyspnea, multivariate analysis found anxiety (P = 0.0318) and maximal inspiratory pressure (P = 0.0187) to be independent correlates of the intensity of dyspnea. Dyspnea is a frequent symptom in patients with advanced cancer. The presence of cancer in the lungs, anxiety, and maximal inspiratory pressure are correlates of the intensity of dyspnea in this patient population. Possible treatments addressing low maximal inspiratory pressure and anxiety are needed, as well as further research in finding new correlates of dyspnea in advanced cancer patients.