Large, randomized clinical trials have long been considered the gold standard to guide clinical care. Metaanalysis is a type of analysis in which results of a number of randomized clinical trials are combined and a summary measure of effect for a given treatment is ascertained. The clinician in practice often is faced with a dilemma regarding the type of evidence that should be used to guide clinical practice; for many clinical problems, there are both randomized controlled trials and metaanalyses available. The cases of calcium and aspirin therapy for the prevention of preeclampsia afford an opportunity to explore the benefits and limitations of each type of study to guide clinical practice. We conclude that, when available, large randomized clinical trials should be used to guide clinical practice.