Last year the Chicago Pride Parade was a huge success, drawing around 750,000-800,000 people and becoming a transportation disaster. This year's was the largest, with 850,000 in attendance, but the city learned from last year, and as Steven Vance chronicles, pulled off the affair in a much smoother fashion.
While looking for something else, I stumbled across video of the Pride Parade from 1977, when it was a considerably more modest affair of 3,000, though it would increase tenfold in the next five years. At least, it seems to be 1977—one astute commenter dates it with an Anita Bryant reference, and what I can make out of the video checks out with the parade route. This seems to make sense, too: "This is before 1978 …why??? BECAUSE THERE IS NO PRIDE/RAINBOW FLAGS!!! which were invented by GILBERT BAKER IN SF '78." Here's an oral history (PDF) of Gilbert.