Few phase III investigations show a benefit for an experimental treatment when compared to a standard therapy or placebo. This illustrates the need for more reliable estimates of treatment effects from the phase II investigations used to design the more definitive phase III trials. In this manuscript, we examine four aspects of phase II clinical trial designs: (1) selecting endpoints; (2) defining the patient population for evaluation; (3) determining a level of activity that would justify a phase III trial; and (4) estimating sample sizes. In each area, problems with the conventional approaches are discussed and alternatives for the successful transition of phase II results to a phase III setting are suggested. An application of the design for patients with androgen-independent prostate cancer is illustrated.