A total of forty four tornadoes (19 in Illinois) struck the area from Missouri and Iowa through Illinois and Michigan during the outbreak, including at least six F3 twisters. A total of 58 people perished. It is the worst tornado disaster in Chicagoland history.
(Below) Oak Lawn High School 
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On a gray and windy day in April 1967, a swarm of tornadoes struck the area, killing 58 people. Oak Lawn was hit particularly hard. The tornado then skipped across the South Side of the city, crossing the Dan Ryan Expressway at rush hour, and hitting the lakefront near 79th Street. Two days later on Sunday, April 23, 1967, three inches (76 mm) of snow fell on Belvidere, which only exacerbated the cleanup from Friday's tornadoes.
My family and I survived, but it was one of the scariest days in my life.
I've never seen another sky quite like the one that day.
Links, images and videos...
Powerful Tornado Devastation, Apr 1967Belvidere Tornado Info
Pictures from the 1967 Oak Lawn Illinois Tornado
Tornado of 1967 and the Spirits that remained
Oak Lawn Tornado Info
More Oak Lawn Info
Lots More Info on the Tornado Outbreak
April 21, 1967. ....a time many of us still remember
Chicago area remembers 30th anniversary of tornado outbreak
Map of April 21, 1967 tornadoes
I very nice PDF download of the weather details and other information
Videos of the disaster, news footage and photos...