Interstate 75 was reportedly so shrouded in haze and smoke from a brush fire and foggy conditions that drivers were virtually blinded. Police said when rescuers first arrived they had to locate victims by listening for screams and moans.
At least five cars and six tractor-trailers were involved, and some burst into flame. Authorities were trying to determine exactly what caused the pileup. Interstate 75 was closed for a time because of the brush fire and was reopened when visibility improved.
Florida Highway Patrol Lt. Patrick Riordan said he was not sure how much time passed between the reopening of the highway and the first crash.
State police estimated the wreckage was strewn for nearly a mile in both directions.
Of the 18 people who needed medical treatment, three of six trauma patients needed surgery, four remained in the emergency room and eight were treated and released.
Northbound lanes of the interstate have been reopened, while southbound lanes remained closed. All six lanes of the interstate - which runs virtually the entire length of Florida - remained closed at midday as investigators surveyed the site and firefighters sprayed foam on the wreckage to put out the last of the fires. It was not clear when the highway would reopen because part of the road melted, Riordan said.
The Associated Press contributed to this report.