The purpose of this study is to determine the extent to which analgesic tolerance develops in chronic pain patients who are either started on opioids or who receive an increase in pre-study opioid dose level and then observed during long-term (20 weeks) stable opioid therapy. In addition to clinical measures of relief of ongoing chronic pain, estimation of tolerance development will be supplemented by periodic assessment of the anti-nociceptive effects of opioids using the brief thermal sensitization model.