Officials say Boston Emergency Medical Services has to take anywhere from two to four patients weighing at least 450 pounds to area hospitals per week.
Capt. Jose Archila told The Boston Globe he's seen patients as much as 700 pounds.
Experts say obese patients can put the health of paramedics in danger, who can injure their backs and necks lifting and moving the overweight.
Paramedic Russ Smith told the Globe he had displaced two vertebrae and strained his back muscles while transporting a woman who weighed at least 400 pounds.
The modified Boston ambulance that hits the streets later this month includes a hydraulic lift.
A stretcher that can bear the weight of 850 pounds, costing $8,000, is also included.
It cost about $12,000 to retrofit the vehicle.