The fire broke out about 2:35 a.m. in the 4700-block of South Princeton Avenue in the city's Fuller Park neighborhood, Chicago police said.
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The two men, ages 45 and 68, were taken to University of Chicago Medical Center in fair and critical condition, according to CPD. Both were suffering from smoke inhalation and one was also burned, Chicago fire officials said.
The 45-year-old man was more seriously hurt.
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When firefighters arrived at the one-and-a-half story building, the flames were intense, but the fire was put out just before 4 a.m.
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The cause of the fire was not immediately available.