Neglected factors in chronic pain treatment outcome studies--referral patterns, failure to enter treatment, and attrition. An increasing number of chronic pain treatment outcome studies have appeared in the literature. In general, these studies support the efficacy of multidisciplinary pain programs, as well as specific treatment modalities such as biofeedback and relaxation. Reviews of this literature have tended to be cautiously optimistic. Some concerns, however, have been raised about the methodological adequacy of these studies, particularly in terms of the lack of control groups, the brief duration of follow-up periods, and the vague criteria used for establishing the success of the therapeutic interventions. Other factors that mitigate conclusions regarding the generalizability of the favorable results reported need to be considered. In this paper 3 rarely discussed topics that are implicit within most treatment outcome studies and that need to be given greater attention are examined. These topics include: (1) referral patterns to pain clinics (who are referred to pain clinics, when, and how representative is the referred sample?); (2) failure to enter treatment (e.g., exclusion criteria, lack of available financial support to cover the cost of treatment, patient's refusal to accept recommendations), and consequently, the representativeness of the treated sample; and (3) patient's attrition. In this paper we discuss each of these factors as they underscore important qualifications that have to be made in evaluating treatment outcome studies. 