Test performance characteristics are important in assessing the clinical usefulness of laboratory tests and serve as a basis for comparing one test to another. Statistical comparisons of performance characteristics are meaningful only when they can detect medically important differences; that is, when they provide adequate statistical power. This requires choosing the appropriate sample size in determining the performance characteristics of interest. Using standard formulas, we designed tables that provide such sample size requirements. Example problems of sample size determination in laboratory test comparisons are given. Used appropriately, this approach should result in better studies of laboratory tests and fewer meaningless negative studies.