(n 23 March 1989, Fleischmann and Pons' claimed they had achieved nuclear fusion /in a jar of water at room temperature. Their claim was met with disbelief mixed with excitement. If cold fusion was possible, it could solve the energy crisis. In the following weeks, physicists from the best research centers in the world sought to replicate Pons-Fleischmann experiment. They failed. On 3 May 1989, while Pons was in Washington waiting to meet with President Bush's advisors, the American Physical Society concluded that every possible variant of the Pons-Fleischmann experiment had been tried without success. The claim of cold fusion was declared invalid. Taubes provided dramatic accounts of the failure of cold fusion in Science2 and elsewhere.3 The cold fusion story shows the importance of replication to the scientific process. Scientists could not replicate the cold fusion claim, and so-regardless of the reputations of the investigators or the journal involved-the claim was rejected. Epidemiologic research is more complex than cold fusion science. Little of the cold fusion story resembles what happens when epidemiologic findings are reported (other than the part about Taubes). To illustrate this, imagine the following scenario. Suppose that in the next issue of EPIDEMIOLOGY, investigators from the Framingham Heart Study were to report that high levels of physical activity after age 40 increase the probability of reaching age 90 by 6-fold. Few epidemiologists would be in a position to replicate this finding. (There are not many follow-up studies with 50 years of physical activity data for thousands of persons.) Even if another such study could be found, would it guarantee replication? What if the study had been conducted in 19th century Singapore? A different outcome could be explained by potential effect modification by diet, or genetic factors, or type of physical activity, or its method of measurement. This is precisely why so many epidemiologic questions remain unresolved: if our study produces a finding that your study does not confirm, we can always find an explanation that you will not be able to refute. We might argue that, say, bedtime can modify the effect of interest and that our study was conducted in Spain where people stay up late. Granted, a large number of experimental details may be omitted in the methods section of a typical paper on cold fusion, but the sheer number of design and analytic decisions and the number of variables that need to be accounted for in a typical epidemiologic study dwarf that in the cold fusion experiments. No wonder epidemiology frustrates policy makers.