OBJECTIVES:  To explore and quantify the extent to which the terms “efficacy” and “effectiveness” are used consistently and correctly in and Health Technology Assessments (HTAs). Efficacy describes a drug’s effect in ideal and controlled circumstances (i.e. in clinical trials).  Effectiveness describes the success of a drug in usual or “real world” practices in which all conditions cannot be controlled. Effectiveness is much more difficult to assess and is often measured by observational studies or calculated by a meta-analysis of clinical trial results.  METHODS:  We examined 38 HTAs published from 2005-2011 covering 13 disease conditions from 6 agencies (AHRQ, DERP, CADTH, IQWiG, NICE, and NHS Scotland), which included 115 pharmaceutical products.  We categorized each HTA based on whether their stated main objective was to measure either clinical efficacy or clinical effectiveness.  These stated main objectives were then compared to the evidence actually evaluated in the reported studies (ie. RCTs and/or observational studies). We quantified and analyzed discrepancies between the stated objectives and actual objectives. RESULTS:  Of the 38 HTAs, 37 evaluated efficacy and 1 focused on effectiveness. Eighteen reviews (47%) described their main objective, efficacy or effectiveness, consistent with the actual evidence evaluated.  Twenty reviews (53%) stated their main objective was measuring clinical effectiveness, but presented evidence assessing clinical efficacy.  Of the 6 agencies, NICE and NHS Scotland showed the highest percentages of discrepancies between stated objectives and evidence evaluated (80% and 100% respectively), while AHRQ and DERP had the lowest (0% and 29% respectively).  CONCLUSIONS:  Though the distinction between “efficacy” and “effectiveness” is substantial, the terms are not always used appropriately or consistently. Often, the uses of the terms in HTAs are misleading. This is a barrier to clear communication, but the implications might be broader.