While establishing himself as one of America's leading jockeys, he was diagnosed with what is now known as type 1 diabetes. Only a few years earlier, before the discovery of insulin, such a diagnosis would have been fatal. Because of his condition, and the nature of diabetes management in the 1930s and 1940s, Woolf had to regulate his weight to avoid rapid dieting. His racing success allowed him to accept only a few mounts each week, riding only 150–200 races each year compared to as many as a thousand rides other jockeys might undertake, and for years he maintained top physical condition. Despite his limited number of races and a relatively short career, Woolf won ninety-seven major stakes races around the United States, including the Hollywood Gold Cup Stakes, the American Derby and the Belmont Futurity Stakes three years running.