At 2 a.m., 911 callers reported a woman screaming for help on the 5000 block of North Rockwell Street, between Winnemac Avenue and Argyle Street.
When investigators arrived at the scene, they found a male victim, stabbed in the neck with a bottle, in an alley behind 2605 West Winnemac.
The offender was described as a Hispanic male, 20 to 30 years old, wearing a white or gray tank top and blue jeans.
The tank top may have been ripped, said police.
The assailant was last seen running eastbound on Winnemac toward Western Avenue.
According to an officer at the scene, the victim was transported to Illinois Masonic Hospital in "serious condition".
UPDATE: Police apprehended a possible offender, covered in blood, near Clark and Winona, at 2:38 a.m.