Self-selection by more food-needy households into the Supplemental Nutrition Assistance Program (SNAP, formerly called the Food Stamp Program) makes it difficult to observe positive effects of the program in survey data. This study investigates self-selection and ameliorative program effects by examining households’ food security month by month for several months prior to initial receipt of SNAP benefits and for several months after joining the program. Two-year panels are constructed by matching the same households interviewed in the Current Population Survey Food Security Supplement in 2 consecutive years using data from 2001 to 2006. Food security is observed to deteriorate in the 6 months prior to beginning to receive SNAP benefits and to improve shortly after. The results clearly demonstrate the self-selection by households into SNAP at a time when they are more severely food insecure. The results are consistent with a moderate ameliorative effect of SNAP—reducing the prevalence of very low food security among recent entrants by about one-third—although they do not conclusively demonstrate that extent of amelioration.