Consumption of omega-3 fatty acid–containing dietary supplements has been purported to reduce cancer risk, but whether this is a valid claim is not known. MacLean and colleagues reviewed published and unpublished clinical studies that have evaluated the effect of omega-3 fatty acid on cancer incidence. The studies reviewed were highly heterogeneous, conducted in 7 countries, used different methods to determine exposure, and assessed effects on 10 different types of cancer. Among 63 estimates of effect reported, only 8 were statistically significant. Overall, the authors found no evidence to suggest a significant association between omega-3 fatty acid consumption and cancer incidence.