For at least 100 years, doctors and self-styled nutrition experts have touted the idea that a particular diet can make cancer treatments work better. Some early clinical trials showed hints of an effect. Now, studies from high-profile labs are spawning a new wave of trials with more rigorous underpinnings. Scientists are unraveling the molecular pathways by which slashing calories or removing a dietary component can make tumors more vulnerable to drugs. In mice with cancer, the effects are sometimes comparable to those for drugs given to patients. Still, compelling results from clinical trials will be needed to overcome some oncologists’ view of special diets as fringy alternative medicine.