Two Massachusetts federal district courts recently addressed—and disagreed about—an important False Claims Act (FCA) issue that has also divided the federal circuit courts: when an alleged FCA violation is based on an underlying violation of the Anti-Kickback Statute (AKS), what kind of causal link must the government or a relator show between the alleged AKS violation and the allegedly false claim for payment? According to the Sixth and Eighth Circuits—and one of the two Massachusetts courts—but-for causation is required. According to the Third Circuit—and the other Massachusetts court—something less will do. Because one of the two Massachusetts courts certified its order for interlocutory appeal, the First Circuit may soon weigh in as well. Regardless of the result, the First Circuit’s decision will likely only add momentum to an issue that already appears ripe for Supreme Court review.