Ronaldo begun his career in 1979, after making the cut during a Corinthians youth tryout. Originally a forward, he was intimidated by the large pool of children who were gunning for attacking positions that day and decided to try out for a goalkeeping position, as he had brought gloves to the try out. Earning a spot as a third goalkeeper with the junior team, Ronaldo was eventually brought to the main team to help out in training. He made his professional debut in a goalless friendly against São José EC in 1988. During that year's Campeonato Paulista he defended a penalty kick from Darío Pereyra in the derby against São Paulo FC, cementing his place as the starting goalkeeper for the team.