The objective of this study was to examine and compare two core measures of Quality Of Life (QOL) used in cancer clinical trials: the European Organisation for Research and Treatment of Cancer QOL Core Questionnaire 30 (EORTC QLQ-30) and the Functional Assessment of Chronic Illness Therapy (FACIT), in order to identify which one patients have the strongest preference for using. 68 patients suffering from Carcinomas of an Unknown Primary site (CUP) were recruited in a multicentric study; all of them completed both questionnaires, administered in a randomised manner. The criteria were the percentage of preferences, and four indicators of acceptability. The results indicated that an equal proportion of patients preferred the QLQ-C30 (19%) and FACIT (19%). 54% of patients felt both questionnaires were acceptable. All the indicators of acceptability favoured the QLQ-C30. Analysis of open-ended questions shed light on the difficulties encountered by the patients. As no significant preference was observed for one of the questionnaires, the QLQ-C30 was chosen on the basis of its significantly better acceptability criteria.