A prospective survey was undertaken to determine the prevalence and characteristics of breakthrough pain in cancer patients admitted to a hospice. Of 414 consecutive admissions, 33 patients were confused or too unwell to take part and 136 were pain-free. The remaining 245 reported 404 pains (range 1-5 per patient); of these patients, 218 (89%) had breakthrough pain and identified 361 pains (range 1-5 per patient). Breakthrough pain was classified as somatic (46%) visceral (30%), neuropathic (10%) or mixed etiology (16%). Thirty-eight percent of pains were severe or excruciating. The average number of daily breakthrough pain episodes was 4 [corrected] (range 1-14); 49% occurred suddenly. Most (59%) were unpredictable, and 72% lasted less than 30 minutes. Seventy-five percent of patients were dissatisfied with their pain control. Breakthrough pain is common among patients admitted to our hospice. It is frequent, short lasting, often unpredictable and not necessarily related to chronic pain making treatment difficult.