The approach of the National Cancer Institute of Canada Clinical Trials Group to measuring compliance, that is, completion rates, for health-related quality of life questionnaires is presented. Completion rates can be measured at the institutional, patient, questionnaire and item levels for baseline, on-treatment and off-treatment follow-up study periods. Time windows are defined for each expected completion time. In seven completed clinical trials, completion rates were high with more than 93 per cent of patients completing questionnaires in the specified time windows at baseline and while on-treatment. The rate while on off-treatment follow-up is still acceptable at 85 per cent. The proportions of analysable questionnaires were 97.6 per cent, 82.0 per cent and 77.0 per cent respectively, at the three study periods. Item completion rates within questionnaires were high at 95.5 per cent or more. The variables most likely to influence baseline and on-treatment questionnaire completion rates were breast cancer, ovarian cancer, metastases and study centre size.