This Mathematica Policy Research, Inc. issue brief was authored by Christopher Trenholm. The Santa Clara County Children's Health Initiative (CHI), launched in January 2001, seeks to extend health coverage to all uninsured children in that California county. This issue brief measures the effect of the initiative on enrollment in Medi-Cal and Healthy Families, the two major State insurance programs. The study found that CHI increased enrollment by almost 13,500 children, or 28 percent. As a result of the gains, CHI increased State and Federal spending in Santa Clara County for the two programs by an estimated $24.4 million during 2001-2002.