From the Economic Policy Institute (EPI):

According to a report released September 16th, 2010 by the U.S. Census Bureau, the number of uninsured Americans under age 65 rose from 45.7 million in 2008 to 50.0 million in 2009. The overall rates for the entire population reached 50.7 million in 2009.

While this country was already in a recession in 2008, the economy sharply deteriorated in 2009.  The unemployment rate increased from 5.8% to 9.3% between 2008 and 2009, the largest one-year increase on record.  As most Americans, particularly those under 65 years old, rely on health insurance through the workplace, it is no surprise that employer-sponsored health insurance fell precipitously from 2008 to 2009.  Employment-based coverage for the under 65 continued to erode for the ninth year in a row, falling 3.0 percentage points from 61.9% in 2008 to 58.9% in 2009.

Read more from EPI: Decline in employer-sponsored health coverage accelerated three times as fast in 2009