I investigate whether a school-based deworming intervention in Kenya had long-term effects on young children in the region. I exploit positive externalities from the program to estimate impacts on younger children who were not directly treated. Ten years after the intervention, I find large cognitive effects—comparable to between 0.5 and 0.8 years of schooling—for children who were less than one year old when their communities received mass deworming treatment. I find no effect on child height or stunting. Because treatment was administered through schools, I also estimate effects among children whose older siblings received treatment directly; in this subpopulation, effects on cognition are nearly twice as large.