Quality of life (QoL) has during recent years become recognized as an important outcome in clinical trials. But it is not sufficient just to incorporate QoL measures in a trial. Several factors have important repercussions on the results of clinical trials using QoL as an outcome measure. Unless more attention is directed towards factors that may be important when evaluating QoL, the value of including these measurements in clinical trials will be minimized. Important issues pertain to trial design, duration of follow-up, the role of placebo effects, side effects, and confounding variables.