An obvious source of information for assessing mental health program effectiveness comes from clients who have received the services and, indeed, much effort has been devoted to developing data collection instruments which are administered to clients upon completion of treatment. To assess the advantages and disadvantages of three treatment follow-up methods, i.e., face-to-face interviews, telephone interviews, and mailed questionnaires, 1100 clients were randomly assigned to one of the 3 follow-up methods, and were scheduled to be contacted approximately 180 days after intake using a well-documented treatment outcome measure. Response rates for the telephone interview and mailed questionnaire were approximately one-half as high as the face-to-face interview; yet for each method very few client background variables were related to whether or not clients responded. The mailed questionnaire was least affected by socially desirable responses and also cost less to administer than either of the interview methods. Given the pressure human service agencies are under to perform follow-up evaluations, the promising results found for the relatively inexpensive mailed questionnaire suggest a need for further research on the external validity of these findings. (Author/JAC)