From the data in the literature it can be seen that 40% of the surgical population has insufficient postoperative analgesia. Many reasons have been given for this: pain control delegated to the doctor on duty and/or the nursing staff; administration of drugs 'on demand', if the patient asks for them, or the nurses feel it to be necessary; fear of causing side effects such as respiratory insufficiency; or provoking addiction by giving narcotics. The aim of this paper is to evaluate the intensity of pain, the side effects, the degree of activity, anxiety, feeling of weakness and the mood of patients surgically treated for oncological diseases of the thorax and upper abdomen, comparing two different antalgic approaches. Thirty-five patients were studied. Pain was treated on demand with a narcotic, or an anti-inflammatory drug, or not treated at all; 20 patients were treated with analgesics given at predetermined hours, following the regime: methadone 10 mg intramuscularly (i.m.) every 12 h from the first to the third day following surgery and sodium diclofenac 75 mg (i.m.) every 12 h from the fourth to the seventh day. Results showed that patients treated with analgesics given intramuscularly at fixed hours have a significantly better pain control during the whole week of treatment (P less than 0.001), on average sleep more (P less than 0.001), spend more time standing or sitting and fewer hours lying down (P less than 0.001), have a higher performance status and feel less weak (P less than 0.05) than the group of patients treated with drugs 'on demand', or not treated at all.