We estimate the size of the U.S. Supreme Court in a world in which the political parties engage in tit-for-tat court-packing. We do so by assuming that the Supreme Court is immediately expanded by four members and that future presidents who court-pack would add enough seats to ensure that a simple majority of justices were appointed by their party. In a series of simulations, we find that median result of repeated partisan court-packing would be to increase the size of the Court to 23 justices within 50 years and to 39 justices within 100 years. We also study the incentives for justices to retire strategically in a world with repeated partisan court-packing and the resulting effects of changes in strategic retirement on the size the Court. We find that court-packing would decrease the incentives for strategic retirement, but we also find that changes in justices’ retirement decisions would have little effect on the eventual size of the Court.