The fire raged for 30 hours. The blaze, leaping from house to house, ultimately destroyed four-and-a-half square miles of Chicago--some 17,500 buildings. By the time the fire burned itself out on October 10, the entire business district was destroyed. Six railroad depots and Marshall Field's department store had gone up in flames. At times, temperatures reached 1,500 to 1,800 degrees. People were incinerated; limestone disintegrated into powder. Some 250 people were known dead and another 200 were listed as missing and 100,000 people were left homeless.

It seems doubtful that Mrs. O'Leary's cow actually started the fire. It seems likely that this myth, which was popularized by a 1938 movie "In Old Chicago," was the product of anti-Irish, anti-Catholic prejudice. Lyrics about Mrs. O'Leary were written to the tune of "Hot Time in the Old Town Tonight." Chicago had thousands of wooden buildings and miles of wood-paved streets and sidewalks to burn. There was a months-long drought that year. In 1997, the Chicago City Council passed a resolution officially exonerating Mrs. O'Leary of blame.

Firefighters were exhausted from battling a 16-hour fire the previous days. That blaze had injured 30 firefighters. To fight the new fire, Civil War General Philip Sheridan mobilized private citizens. When the fire was finally extinguished, he declared martial law and used guards from the Pinkerton Detective Agency to prevent looting.