The assumption that the design of an antidepressant clinical trial affects the outcome of that trial is based on sparse data. We sought to examine if the dosing schedule, either a fixed dose or a flexible dose type, in an antidepressant clinical trial affects the frequency with which antidepressants show statistical superiority over placebo. Randomized, placebo-controlled clinical trials of nine antidepressants approved by the Food and Drug Administration between 1985 and 2000 were reviewed. These trials comprised 9313 depressed patients who participated in 51 antidepressant clinical trials consisting of 92 treatment arms with eventual approved doses. In the flexible dose trials, 59.6% (34/57) of the antidepressant treatment arms were statistically significant compared to placebo, whereas in the fixed dose trials only 31.4% (11/35) of the antidepressant treatment arms were statistically significant compared to placebo (chi(2)=6.9, df=1, p<0.01). These data suggest that the antidepressant dose schedule may influence trial outcome due in part to a significantly lower magnitude of symptom reduction with placebo in flexible dose trials (F=4.08, df=1, 48, p&<0.05) compared to fixed dose trials. Symptom reduction was similar with antidepressants in the flexible and fixed dose trials. Further, the primary function of finding a dose-response relationship was not found among the fixed dose studies.