We study the dynamics of participation and health care consumption in the Affordable Care Act’s health insurance marketplaces. Unlike other health insurance contexts, we find individuals commonly drop coverage midyear — roughly 30% of enrollees exit within nine months of sign-up. While covered, dropouts spend more on health care than in the months before sign-up or after exit. We model the consequences of drop-out on equilibrium premiums and consumer welfare. While dropouts generate a type of adverse selection, the welfare effect from their participation is ambiguous and depends on the relative costs per month of part-year vs. full-year enrollees. In our empirical setting, we find that imposing a penalty that incentivizes participation for at least 3.5 months would lower premium levels and improve overall consumer welfare.