The five most expensive conditions for hospital treatment in 2013 were septicemia; osteoarthritis; care for newborn infants; complication of device, implant, or graft; and acute myocardial infarction.
These conditions accounted for about 20% of total hospital costs.
For more info see Torio and Moore (2016). National inpatient hospital costs: The most expensive conditions by payer, 2013. Rockville, MD: Agency for Healthcare Research and Quality.