Much of the money made by Bradley during his time in Siam was from performing medical services for the native people and missionaries. Bradley was perceived as a "Great American Doctor" and had immediate credibility among the people of Siam. Bradley challenged many of the old medical practices of the country, especially women lying by fire after pregnancy for a month, and wrote multiple books on these topics so that native doctors could learn of Western medical practices.
Bradley is credited with performing the first surgery in Siam, removing a cancerous tumor from the body of a slave. After this surgery, Bradley became highly sought after for medical advice from the royal court. While it took a fair amount of time, the royal court gained trust in Bradley and called on him for medical advice for years. Bradley taught royal doctors how to perform the same practices as he did and he wrote numerous books for the purposes of the court.
Bradley's greatest medical challenge while in Siam was attempting to produce a vaccination for the smallpox virus, which devastated the country and Bradley, killing his eight-month-old daughter, Harriet. Bradley received many trial vaccinations from Boston, none of which were successful. Bradley solved this problem by using the inoculation technique. (The inoculation technique was documented as having a mortality rate of only one in a thousand. Two years after Dr. Peter Kennedy's description appeared, March 1718,[2] Dr. Charles Maitland successfully inoculated the five-year-old son of the British ambassador to the Turkish court under orders from the ambassador's wife Lady Mary Wortley Montagu, who four years later introduced the practice to England.)[3] Seeing the success that Bradley had with the technique, the royal court once again called on Bradley to vaccinate their children as well as many natives and slaves.