In recent years the necessity of including quality of life (QL) measurement in cancer research has been stressed. In this paper an overview is given of the results of studies into the QL of cancer patients. From descriptive studies it appears that the quality of certain domains of life is impaired by cancer treatment. Results from studies in which two or more groups of cancer patients are being compared are not consistent. The expectation that the QL of patients is impaired more negatively by certain treatment modalities is confirmed in some studies but not in several others. Even the assumption that the QL of cancer patients is worse than the QL of the normal population is not substantiated. In this paper explanations for these unexpected results are forwarded. First, the definition and operationalization of the concept QL differs from one study to another. QL may either refer to an overall evaluation or to the evaluation of certain domains of life, and, either to the subjective experience of the patient or the evaluation of the situation by others. Secondly, other methodological difficulties especially with respect to reliability, validity and design are described. Finally, it is suggested that psychological mechanisms may account for the absence of differences between cancer patients and others and may therefore, on theoretical grounds, explain the established inconsistencies.