Since the early 2000s, foster care caseloads have decreased in many wealthy democracies, yet the causes of these declines remain, for the most part, a mystery. This paper uses administrative data from one country that experienced a sharp decline in foster care caseloads, Denmark, to show that increasing medical treatment of ADHD (primarily through the use of Ritalin) accounts for a substantial share of the decrease in foster care caseloads. According to our estimates, the decline in foster care caseloads over this period would have been 45% smaller absent increases in medical treatment of ADHD. These findings are provocative in light of recent research showing ambiguous effects of medical treatment of ADHD on children once the share of children treated exceeds a certain threshold. While a host of factors such as parental behaviors and characteristics, welfare generosity, and the female imprisonment rate all shape foster care caseloads, future research should be attentive to how medical treatment aimed at addressing children’s acute behavioral problems could also have a powerful effect on foster care caseloads.