Crystal Lake Fire Rescue Chief Paul Deraedt says firefighters responded to the blaze at the Valley View Acres horse boarding facility in unincorporated Woodstock shortly before 11 p.m. Saturday.
He says only five of the 37 horses in the stable are believed to have survived the fire, but they appear to be in good condition.
Valley View Acres' website says it also breeds and trains a variety of horses, including thoroughbreds and show jumping horses.
Deraedt says preliminary estimates put the total value of the dead horses at more than $1 million and that each was likely insured for about $50,000 to $75,000.
The cause of the fire is being investigated.