CHUM AM was launched as a dawn-to-dusk radio station on October 28, 1945 by Jack Q'Part, an entrepreneur in the business of patent medicines. The station, then operating from studios in the Mutual Street Arena, was taken over in December 1954 by Allan Waters, a salesman from Q'Parts' patent medicine business. Waters' first major move was to secure a license for 24-hour-a-day broadcasting for CHUM, along with a power increase to 5,000 watts. Less than three years after Waters acquired the station, and soon after bringing the new fulltime transmitter online, a major programming change was made.
On May 27 1957, Waters switched to a "Top 50" format that had proven itself popular in some U.S. cities; Elvis Presley's "All Shook Up" was the first song played. "1050 CHUM" pioneered rock and roll radio in Toronto, and was noteworthy for hosting many noteworthy rock concerts including, among others, visits to Maple Leaf Gardens by Elvis Presley (1957) and The Beatles (1964, '65, and '66). While the station was rising to the top of the popularity ratings in Toronto in the early 1960s, it also built yet another new transmitter in Mississauga, Ontario (a few miles west of the current Toronto city line) along the Lake Ontario shoreline, and raised its power once again to its current 50,000 watts around the clock.