The role of Vespucci has been much debated, particularly due to two of his letters of which the authenticity has been brought into doubt: the Mundus Novus (New World) and the "Lettera" (or "The Four Voyages"). While some have suggested that Vespucci was exaggerating his role and constructing deliberate fabrications, others have instead proposed that the two letters were forgeries written by others of the same period.
It was the publication and wide-spread circulation of his letters that led Martin Waldseemüller to label the new continent "America" on his world map of 1507. He wrote that he based it on the Latinisation Americus Vespucius, taking the feminine form America (see naming of America).
The two disputed letters claim that Vespucci made four voyages to America, while at most three can be verified from other sources. It is now generally accepted by historians that no voyage was made in 1497 (which allegedly began from Cadiz on May 10 of that year). Little is known about the final voyage.
In 1499-1500, Vespucci joined an expedition led by Alonso de Ojeda. After hitting land at the coast of what is now Guyana, the two seem to have separated. Vespucci sailed southward, discovering the mouth of the Amazon and reaching 6°S, before turning around and seeing Trinidad and the Orinoco River and returning to Spain by way of Hispaniola.
His next voyage in 1501-1502 was in service of Portugal. The leader of this expedition is not known. On this voyage he sailed southward along the coast of South America. If his own account is to be believed, he reached the coast of Patagonia before turning back.
Vespucci's real importance for history may well not lie in his discoveries per se, but in his letters, whether or not he wrote them all himself. From these letters, the European public for the first time got information about America, thus popularizing the subject.