A simple cost function approach is proposed for designing an optimal clinical trial when a total ofNpatients with a disease are to be treated with one of two medical treatments. The cost function is constructed with but one cost, the consequences of treating a patient with the superior or inferior of the two treatments. Fixed sample size and sequential trials are considered. Minimax, maximin, and Bayesian approaches are used for determining the optimal size of a fixed sample trial and the optimal position of the boundaries of a sequential trial. Comparisons of the different approaches are made as well as comparisons of the results for the fixed and sequential plans.