The Affordable Care Act's employer mandate requires large firms to pay penalties unless they offer affordable health insurance coverage to full-time employees, raising concerns that employers might lay off workers or reduce hours. In this brief, we estimate the number of workers potentially at risk of losing their jobs or having hours reduced. Most workers near the thresholds--those in firms with around 50 full-time-equivalent employees or those working near 30 hours per week--are already insured or have been offered coverage. There are 100,000 full-time workers at the firm-size threshold and 296,000 at the hourly threshold who are uninsured. Fewer than 10 percent, less than 0.03 percent of the U.S. labor force, might see reductions in employment or hours in the short run. Over time, employment patterns might change, leading to fewer firm sizes and work schedules near the thresholds, potentially affecting up to 0.5 percent of the workforce.