The UK Medical Research Council (MRC) Daily Diary Card for quality of life (QOL) assessment was first designed over ten years ago, and since then has been in continuous use in cancer clinical trials organized by the MRC Cancer Trials Office. However, clinical trials using this technique have only reached maturity since 1989 and later. Thus it is timely to review the experiences gained by use of this method of assessing QOL. Results from a series of clinical trials confirm that the Daily Diary Card appears to be a valid, reliable and sensitive instrument which is able to detect the transient changes in health and QOL which occur on a day to day basis during therapy for cancer. However, in early studies we also encountered a number of problems which were addressed during later MRC trials; we believe that many of these are of general importance to any investigation of QOL in patients, many of the issues raised remain controversial, and the practical lessons learned from applying the Diary Card will be of relevance to those planning future studies of QOL assessment.