Our aim was to determine the impact of three different firefighting uniforms (traditional, modern, and modified modern) on the incidence and severity of thermal burn injuries, the major occupational injury affecting firefighters. Injury data were collected prospectively for the entire New York City Fire Department (FDNY) firefighting force wearing FDNY's traditional uniform (protective over-coat) from May 1, 1993 to August 31, 1993; FDNY's modern uniform (protective over-coat and over-pant) from May 1, 1995 to August 31, 1995; and FDNY's modified modern uniform (short sleeved shirt and short pants, rather than long-sleeved shirt and long pants, worn under firefighter's protective over-clothes) from May 1, 1998 to August 31, 1998. Outcome measures were burn incidence and severity. Adverse outcomes were heat exhaustion and cardiac events. During this 12-month study, 29,094 structural fires occurred. The incidence rate for upper extremity burns was 2341 per 100,000 fires and for lower extremity burns, 2076 per 100,000 fires. With the change from the traditional to modern uniform, the distribution of burns per fire decreased significantly (P = 0.001) for upper extremity burns (86%) and lower extremity burns (93%). With the change from traditional to modern uniform, days lost to medical leave for upper or lower extremity burns decreased by 89%. The majority of burns occurred at the lower arm and mid-leg, and the change to the modern uniform decreased such burns by 87% and 92%. Burn incidence and severity were not significantly affected by the change to the modified modern uniform. The distribution of heat exhaustion or cardiac events per fire was not significantly affected by the change from the traditional to modern uniform, and heat exhaustion was decreased (P < 0.001) by the change to the modified modern uniform. In conclusion, the modern uniform dramatically reduced burn incidence and severity without adverse impact. The modified modern uniform significantly reduced heat exhaustion without significantly affecting thermal protection.