The extent to which the results of a study can be attributed to the intervention under investigation(i.e., internal validity) is an important consideration in interpreting study findings. There are many threats to the internal validity of designs frequently used in medical education research. Synthetic designs, which involve the integration of two or more weak designs, or the addition of design elements, may afford investigators greater control over confounding variables in medical education research. A rationale for using synthetic designs is presented and two examples of their use in medical education settings are examined. The concluding proposition is that synthetic designs allow investigators flexibility in planning research that is feasible in medical education settings. In addition, they may permit stronger causal inferences between interventions and results than traditional research designs.