Cancer pain can be effectively controlled in most patients by classical pharmacological treatment. We retrospectively studied the characteristics and factors associated with non responsive pain. Between 1989 and 1996, 1767 patients were referred to our pain center; 831 (47%) had cancer pain and from 787 evaluable cases 118 (15%) experienced non-controlled pain whereas good pain control was achieved within a few days in 669 (85%) patients. Gender, age, cancer type, metastasis, initial pain intensity, nociceptive or neuropathic components and administration of adjuvant therapies were similar in both groups. On the other hand, diffuse pain, abdominal pain, terminal care, near death and doses of strong opioids were significantly different. Factors associated with therapeutic failure were conflicts, life and complications and breakthrough pain. In the presence of refractory cancer pain the factors predictive of therapeutic failure should be identified in order to optimize individual pain treatment.