Outcome and economic studies pertaining to the use of therapeutic drug monitoring (TDM) are summarized. The studies were classified by a 2 × 2 matrix comprised of process, outcome, system-related variables, and patient-centered variables. Two hundred forty-seven studies conducted from 1974 to 1994, culled from eight summary sources, were reviewed. Almost 75% of the studies reported system-related, not patient-centered, measures for assessing the value of TDM; nearly the same percentage evaluated processes as opposed to outcome measures. Studies comparing TDM to non-TDM populations generally showed that TDM as an intervention reduced the rate of undesirable system-related variables by 50% and increased the rate of desirable system-related variables by 100%. For patient-centered variables, TDM was less dramatic, decreasing the rate of undesirable variables by 15% to 50%. Only 8% of the studies examined economic variables. TDM-influenced changes in process variables resulted in a projected mean annual savings of $37,000. The few studies available of cost-benefit analysis focused on aminoglycosides and showed that TDM yielded a range from 4:1 to 52:1 in benefit-to-cost ratio. Most studies have limitations that include small sample size, short study periods to predict long-term outcomes, no long-term follow-up, and designs for noncontrolled study.