OBJECTIVES:  We previously highlighted the importance of disease-specific data in understanding growth in healthcare spending.  This update presents more recent data and decomposes the drivers of spending growth into price, utilization, and disease prevalence changes. METHODS: US employer health care spending was estimated using claims data from employer sponsored health plans from 2004 and 2009 (to be updated to 2010 for presentation).   Claims were assigned to disease-specific episodes using the Thomson Reuters Medical Episode Grouper (MEG).  Data were projected using sampling weights derived from the Medical Expenditure Panel Survey and 5-year growth in spending was decomposed into growth in disease prevalence, resource consumption per disease, and medical care price level.  RESULTS: Annual spending data were analyzed for 107 employers with 11.5 million health plan members in 2004 and 143 employers with 18.0 million members in 2009.  Over these 5 years spending per member per year grew 34% overall, 11% after inflation adjustment.  The top 50 disease categories (out of 560) accounted for 52% of spending in 2009.  Prevalence of disease-specific treatment grew for 42 of 50 categories, led by obesity (93%), renal failure (91%), and complications of medical and surgical care (43%).  Overall, growth in prevalence drove more than 90% of the inflation-adjusted spending increase.  Resource use per patient grew in 27 of the top 50 diseases, led by dyslipidemia (106%), multiple sclerosis (57%), and osteoarthritis except spine (42%).  However, other diseases showed declines and overall intensity of resource use grew only 2% over 5 years. CONCLUSIONS: Apart from medical care price inflation, the major driver of growth in employer health care spending is increased prevalence of treated disease, led by obesity and its consequences.  A secondary factor is increased intensity of resource use in those diseases targeted by innovation in biologic therapies and surgical devices.