This report reviews the 100 most recently approved drugs in order to quantify the frequency with which post-marketing studies of drug efficacy can be performed experimentally and non-experimentally. These drugs represent 131 potential drug uses. Of them, the absolute efficacy of 89 (68 percent) could be evaluated from clinical observations. Of the remaining 42, six (14 percent) could be studied experimentally or non-experimentally, six (14 percent) only experimentally, one (2 percent) only non-experimentally, and 29 (69 percent) by neither technique. Answers to all questions of relative efficacy required formal research. Of these, 94 (72 percent) could be studied using either experimental or non-experimental techniques. The remaining 37 (28 percent) could be studied experimentally only. Thus, clinical observations and non-experimental research can contribute a large proportion of the information about drug efficacy still needed after marketing.