One of the main issues to be considered in conducting clinical trials concerns the presence of missing data. This aspect is particularly relevant in oncology longitudinal studies, characterized by a long follow-up, and especially in quality of life studies where there is still little knowledge about patients' characteristics that predict loss of data. Since the middle of the: 1980s the GIVIO (Interdisciplinary Group for Cancer Care Evaluation) co-operative group has been involved in conducting quality of life assessment studies, also focusing on the development of some strategies aimed at the minimization of missing data. In this paper we report on the results of two trials, which are now completed, concerning the quality of life assessment in a sample of breast and colon cancer patients. In order to cope with the problem of missing data, in both the trials the strategy of follow-up mailing was adopted, which proved to be an effective way to increase the response rate by nearly 50 per cent at each time point. (C) 1998 John Wiley & Sons, Ltd.
