30-40% of cancer patients suffer from pain at diagnosis while 70-80% of patients at progressed stages of the disease suffer from pain. Background pain is treated with long-acting opioids. Breakthrough pain can be treated with shorter acting non-opioid analgesics or opioids. The aim of this study was to describe the medical treatment of pain in cancer patients in connection with six Danish hospital units with special expertise in pain treatment. Differences in the prescription of analgesics were studied. The study was performed as a cross section study of prescribed analgesics. Data was collected by reviewing medical records. The study included 347 patients. A total of 278 patients out of 347 were treated with opioids for background pains. A significant difference was found (P < 0.001) in the frequency of prescribing morphine, oxycodone and fentanyl. For the treatment of background pain secondary analgesics were prescribed for 40% of the patients while 50% of the patients were treated with paracetamol and/or NSAID. According to the medical records 79% of the patients were prescribed analgesics for breakthrough pain. 73% of the 347 patients had strong opioids prescribed for breakthrough pain. For the treatment of background pain opioids were prescribed for the majority of the cancer patients. Morphine and oxycodone were prescribed most frequently. Secondary analgesics and paracetamol and/or NSAID were also prescribed for background pain. The strong opioids were prescribed for the treatment of breakthrough pain. Differences in the prescription of analgesics between the six hospital units were observed in this study.