The purpose of this study was to examine hospitalised cancer patients' and nurses' assessment of patients' cancer pain and to compare them. The data were collected from 51 patient-nurse pairs in two hospitals from oncological and medical clinics. Each nurse and patient took part in the study no more than once. The data were collected with a structured interview and the questionnaire. The intensity of pain was measured with a visual analogue scale (VAS) and the Finnish version of the McGill Pain Questionnaire (FPQ). The results showed that the differences between patients' and nurses' assessments were statistically significant for most intensive pain and for acceptable pain. In both cases nurses' assessments of the intensity of pain were lower than patients' assessments. The nurses identified 40 words in the verbal FPQ that the patients used in describing their experiences of pain. The words used most often by patients were agonizing, tender, wave-like and radiant. The word that the nurses used most often was that of intense. Nurses' knowledge about pain medication in general and morphine in particular was clearly associated with the differences observed in estimates of the intensity of pain. Nurses with poor knowledge underestimated the patients' most intensive experiences of pain. The difference was statistically significant.