Human resources (HR) business process outsourcing (BPO) is the fastest-growing segment of HR services and will grow at a compound annual growth rate (CAGR) of 16% in the United States, reaching $18.9 bln in spending by 2010. In 2004, 37% of US HR outsourcing spending was on HR BPO services. IDC projects that HR BPO services will continue to grow, reaching 46% of HR outsourcing spending in 2010 as the market solidifies and leaders achieve profitability through economies of scale.