This article discusses some of the issues involved in the use of quality of life methods to evaluate treatment, primarily in trials of cardiovascular disease. The choice of aspects to measure is influenced by the severity and nature of the disease, the expected benefits and adverse effects of treatment and pragmatic considerations such as the length of the trial, and availability of the methods. The choice of specific instruments should take account of good measurement criteria such as validity and reliability, and, most importantly, sensitivity to the effects of treatment. The advantages and disadvantages of interviewer and self-completed questionnaires are discussed. The choice of respondent is primarily the patient. In some circumstances it may be appropriate to also include family members, either for their views on the patient, or for an assessment of their own quality of life as this may also be disrupted. Withdrawals are a major problem in the analysis of all trial results. The inclusion of a Health Index is one method of obtaining quality of life scores from patients who withdraw or die. The interpretation of quality of life results from trials is not simple, for example, the benefit of trial inclusion needs to be distinguished from the effect of treatment. Moreover, changes in quality of life scores should correspond to changes in the patients’ own experiences of their daily lives.